Grammarly is designed for amateurs, and amateurs are inclined to take its suggestions at face value, then argue them as gospel.

I hate Grammarly the same way I hate Strunk & White. You and I can run our work through automated tools to flag the errors we got too close to to see. I prefer the current iteration of Microsoft Word’s built-in editor tools, but they, too, used to be hideously bad and still aren’t perfect. Nevertheless, you and I understand the underlying rules and can make informed judgements as to whether they apply. They are dangerous in the hands of non-native-English speakers, in the same way a thesaurus is.