Well, I should really qualify that I’ve never used Grammarly per se, either. But as a (now retired) contract technical writer, I did work in a couple of places where I had to have my writing approved by an algorithmic grammar checker before publication. So I don’t know, maybe Grammarly does suck less than those earlier generations of tools.

But just as you say, I find the whole concept demeaning and offensive. I learned my grammar in school, back in the day when they actually taught such things. I know a gerund from a present participle and an adjectival prepositional phrase from a relative subordinate clause. I don’t need no stinkin’ robot to correct my sentences.

(That last sentence, of course, would never have gotten through the robot grammar police.)