This is an interesting study, but the conclusion it appears to draw is erroneous at best.
Take this quote, from one of the study's investigators: "During sleep, our neurons are busy doing very complicated processing, including, this study shows, generating sleep spindles to protect us from being awoken from noises in the environment."
EEG is such a broad average that it tells us very little about what the brain is doing, just like looking at the NASDAQ doesn't tell you very much about how one company or a group of companies are doing. To suggest that our brain is "generating sleep spindles" is myopic; sleep spindles are a symptom of what the brain is doing during sleep: replaying memories temporarily held in the hippocampus and consolidating then into cortex.
The correlation between producing lots of "sleep spindles" and having relatively good memory makes sense in this light, as does being hard to wake up during sleep, as a brain that's attending to memory consolidation won't be as sensitive to external stimuli (just like when you're concentrating while conscious). But to suggest that sleep spindles function to protect us from noises in the environment makes no sense at all. Evolutionarily, it's more advantageous to wake up when you are being attacked, or are otherwise in peril. If anything, this research would suggest some kind of limiting factor to the overall intelligence of a society that deals with the environment in that way.