Trump’s grammar in speeches ‘just below 6th grade level,’ study finds
Because his grammar is better than George W. Bush's, yep, it's official. Trump with 6th grade and Bush W. was 5th grade.
Though his vocabulary is the worst. But Reagan man, his was damn good.
Reading your link seems to contradict your conclusion. A speech is written by speech writers, those skilled in putting words on paper geared to the audience.
A better measure is the vocabulary and syntax when speaking to others off the cuff. Sometimes the speaker has a ready response which they use whenever required to do so, others obviously think as they speak and use fillers ("you know", "Um"), and some are natural contemporaneous and extemporaneous speakers, those who have studied myriad issues and thought them through.
I think those examples - sans Lincoln - we have heard speak without preparation and are clearly different. trump relies on fillers, rote comments and deflections; Obama's thinks as he speaks, thus the stutter, he is concerned with being precise, GWB was, well GWB, Bill Clinton was the most accomplished speaker and likely the smartest, and Reagan the most polished who could charm the rust off of a squeaky gate.