On the other hand, if turnitin's new grammar grader turns out to be accurate,
Word is terrible for grammar checks. What is the expectation that Turnitin got the grammar right?
Even if they get it right, it won't work at my SLAC, as it is perfectly possible to write an essay with no grammatical errors that still receives a failing grade. There's a whole lot more to write effectively than grammar.
Oh, I get to have that argument routinely as well. Yes, you wrote a grammatically correct essay. Yes, it has a visible logical structure. Yes, it has all the vocabulary words expected, has the right number of references, and has the correct format. It's still MadLibs-like word salad that is garbage. F.