TurnItIn is also easy to misconfigure and generate tonnes of false positives, e.g. when basing your work on someone else's research from which you've quoted, or it thinking you've plagiarized your references section because you've referenced exactly the same texts as millions of other people.
When publishing on Amazon now, there's a radio button you have to select to declare if you used AI software to write your book. So they can 'collect information about the use of machine tools in creating content'. To what end, I'm not sure.
So called "AI" generators are very good at churning out content that's the same as everyone else's, because that's all they have to go on: a huge body of already published work to steal from, tweak and pretend it's new.
It's even more hilarious when asking it to generate product or service blurb for a company: it spits out the same meaningless platitudes and drivel that almost every lazy copywriter has ever written on the web or copied from other websites. Again, because that's all it can do. It's not machine learning, it's machine regurgitation.
As for Grammarly being classed as AI, that's an intriguing classification given it's likely crowdsourced data that's used to make its largely piss-poor suggestions. Anyone who blindly goes through and clicks Yes to everything it suggests will, at best, have their work sandblasted to sound like everyone else's (as if an overbearing editor made the voice of your book sound like theirs) and, at worst, it'll ironically contain grammatical errors or awkward sentence constructs because it hasn't understood your intent.
I appreciate it can be helpful as a fallback to some, but I'm glad to have deleted that bag of shite off my devices!