this claim is totally bogus. If it was true, half my sites written by non native english writers would have been pandalized as well. Just because an article was not written by someone with a PHD means its not useful?
There are tons of USEFUL content that is not textual. Some people are just not advanced, its sometimes actually better to write on a basic level that everyone can understand. There are too many poorly written sites still ranking to even think google uses this as a major factor.
Google wants us to believe they are way more advanced than they really are, but the reality is this panda update is not about 1 thing and the writers ability to write is not a major factor if it is one at all.