This is one of those slippery slope things where Grammarly did "just" Grammar and then slowly got into tone and perception and brand voice suggestions and now seems to more or less just want to shave everything down to be as bland as possible.
I tried using an LLM to help me write some stuff and it simply didn't sound like I'd written it - or, it did but in a kind of otherworldly way.
The only way I can describe it is like when I was playing with LPC10 codecs (the 2400bps codec used in Speak'n'Spells, and other such 80s talking things). It didn't sound like me, it sounded like a Speak'n'Spell with my accent, if that makes sense.
No? Okay, if not, if you want I could probably record another clip to show you.
All you have to do is prompt your AI with a writing sample. I generally give it something I wrote from my blog. It still doesn't write like I do and it seems to take more than that to get rid of the emdashes, but it at least kicks it out of "default LLM" and is generally an improvement.