I agree with this. Unless I'm looking for something regarding perfect English, the seller's language skills wouldn't concern me. And if they were honest and said, "Native is 'so and so' language" and added English as an additional language, then I'd think 'wow, that's pretty cool that they know more than one language'. I'd also ensure that when I placed the order that my English was impeccable and even if they made a mistake I wouldn't get down on them because learning a second language is hard- I'm doing it, and I get it. BUT, if they claim to be native speakers and their English isn't up to par; that's a different story.
His phrasing could definitely improve, but I understand the sentiment. I do editing, and I have this written in my GIG description because a lot of times, I've encountered clients who don't know the different types of editing. So they'd purchase or enquire about one type when they're really looking for another.