Does not sound like much of a study. More like a bit of a theory.
Yep. Researchers find a trend in the data, then rationalize an explanation and present it as "theory".
I'll propose an alternate explanation for the data.
People are tired of being told what to think, the outlets have been telling people what to think in the strongest possible terms, and as a result the strength of the words has declined.
Calling someone a liar, fascist, racist, islamophobe, Hitler, Cthulhu, and everything else was so completely over the top(*) that many people simply got used to the terms, thinking that exaggeration was the new normal they applied an internal reverse bias to compensate.
The term "sad" is mild, so when you encounter it you might think the person saying it *isn't* exaggerating, and may be choosing their words carefully. It's the difference between someone saying "I'm uncomfortable" versus "I'm too hot!". Literally, the 2nd phrase implies required action, which isn't usually true (that the action is required), and is taken as exaggeration. The 1st phrase sounds more accurate and reasonable, and gives the impression of truthfulness.
So when Trump says something is "sad", it's in lieu of calling something bad, nasty, stupid, or unconscionable. It comes off as more nuanced, non-exaggerated, and more trustworthy.
That's my theory, and it also fits the data.
Can someone propose a test to distinguish between the two theories?
(*) If you don't think that the recent media coverage was over the top, consider Breitbart's enormous jump in readership in recent months, [Democrat minority leader] Nancy Pelosi is desperately trying to shore up support, and CNN is now literally synonymous with the term "fake news". That doesn't happen overnight, nor from isolated events, nor does it happen for no good reason.