If I think you're fluent, I might think you're an idiot when really you just don't understand.
If I know they struggle with English, I can simplify my vocabulary, speak slower/enunciate, and check in occasionally to make sure I'm communicating in a way they can follow.
Both of those options are exactly what the writer wants to avoid though, and the reason they are using AI for grammar correction in the first place.
Thank you for demonstrating my point.