Actually the book was from 1988, and uses a huge set of research.
Also, rote memorization was the research topic as such because it seeks to push your brain's memory functions directly, rather than train techniques. That's why research showing improvement has gone on to discover subjects which improved had developed memory systems, not made their brains stronger by flexing them repeatedly.
Finally, let's excerpt from your paper:
Participants were randomly assigned to 1 of 4 groups: 10-session group training for memory (verbal episodic memory; n=711), or reasoning (ability to solve problems that follow a serial pattern; n=705), or speed of processing (visual search and identification; n=712); or a no-contact control group (n=704). For the 3 treatment groups, 4-session booster training was offered to a 60% random sample 11 months later.
So far, so good.
Memory training focused on verbal episodic memory. Participants were taught mnemonic strategies for remembering word lists and sequences of items, text material, and main ideas and details of stories. Participants received instruction in a strategy or mnemonic rule, exercises, individual and group feedback on performance, and a practice test. For example, participants were instructed how to organize word lists into meaningful categories and to form visual images and mental associations to recall words and texts. The exercises involved laboratory like memory tasks (eg, recalling a list of nouns, recalling a paragraph), as well as memory tasks related to cognitive activities of everyday life (eg, recalling a shopping list, recalling the details of a prescription label).
The memory training participants were taught new techniques. This is skill, not brute force. If you did push-ups exactly the same way, you'd get bigger muscles; but this is teaching people to do those push-ups by moving their hands to a correct position which requires less effort and more efficiently lifts the body.
Reasoning training focused on the ability to solve problems that follow a serial pattern. Such problems involve identifying the pattern in a letter or number series or understanding the pattern in an everyday activity such as prescription drug dosing or travel schedules. Participants were taught strategies to identify a pattern and were given an opportunity to practice the strategies in both individual and group exercises. The exercises involved abstract reasoning tasks (eg, letter series) as well as reasoning problems related to activities of daily living.
Reasoning training was based on teaching techniques to analyze and approach problems. Again, technique. This is like learning about Kepner-Tregoe problem analysis.
Speed-of-processing training focused on visual search skills and the ability to identify and locate visual information quickly in a divided-attention format. Participants practiced increasingly complex speed tasks on a computer. Task difficulty was manipulated by decreasing the duration of the stimuli, adding either visual or auditory distraction, increasing the number of tasks to be performed concurrently, or presenting targets over a wider spatial expanse. Difficulty was increased each time a participant achieved criterion performance on a particular task.
K. Anders Ericsson explains something called the "OK Plateau". Most people learn initially by cognitive effort, and then internalize that into autonomous task: it moves from activating the prefrontal cortex to activating the basal ganglia. At a point, people subconsciously decide they're doing good enough, and cease improving.
Ericsson outlines three strategies experts use. Deliberate focus brings the task into cognitive recognition; goal-oriented behavior demands improvement; and immediate feedback points out current performance so the experts can analyze and adjust for their shortcomings.
Having trained myself in speed-reading, I can relate to the speed-of-processing study. I've had to deliberately focus on the RSVP, analyzing my own cognitive process. Initially, my mind would mill over words, return back to words I'd read, and stop focusing on what I was reading. This can be done between words in free time to rebuild and reanalyze, but not for extended blocks of 1-2 seconds when RSVPing at 450-800 words per minute. My mind also tends to wander to other related thoughts--which I had to stop.
By increasing speed, the researchers demanded additional focus. By adding distractions, the researchers demanded improved filtering of distractions specifically (rather than just internal thought). These changes largely demand the subject improve focus, accept a certain error rate, and employ strategies to maximize recognition of the most information in the least time. When multiple cognitive tasks are present, the subject must recognize the recognizable information so as to attend to it first, and move to the less-recognizable once the delay in processing won't cost so much (diminishing returns); when multiple, time-sensitive tasks are presented, rapid prioritization becomes important.
This particular part of the research provided an environment in which direct focus was enforced, goals were obviated, and immediate feedback was provided. Pattern behavior would obviously develop from such a strict environment, up to physiological limits.
None of that research says the brain bench pressed a bunch of information and became stronger and tougher. It suggests skill development, or at least suggests the strong possibility of skill development. My above discourse about cognitive processing skills is an implied likelihood not addressed by the paper; while the paper itself specifies the teaching of specific, researcher-selected mnemonics and problem-solving skills, rather than the exercise of basic mental faculties.
Nothing in there suggests the brain is a muscle and benefits from exercise. Much of that directly references technique, while the remainder supplies a situation where technique could easily develop and would be useful. I would bet money that tasks requiring similar cognitive effort and load on the same mental faculties, yet wholly unaided by any technique which could improve any of the things tested, would show zero improvement after the experiment.