EL&U: English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts.
ELL: English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English.
The two taglines don't seem to properly represent what has been going on on both EL&U and ELL. Many questions on EL&U are just cr** (Low Quality Questions) and some good questions on ELL seem to belong to EL&U. There is a grey area where distinction is not clear enough between EL&U and ELL.
I don't think the ELL's tagline necessarily meanmeans this community is exclusively for non-native speakers, especially in terms of answering questions. As Varun KN mentioned, native speakers could add value that non-native speakers sometimes can't and it is a great advantage to have dedicated native speakers who can check and balance any errors or mistakes that could be made by non-native speakers.
I think most of the native speakers on ELL are helpful and sincere in terms of helping non-native speakers and learners.
I am not perfect in my native language, nor am I in English. The important thing is I learn new things every day. There are new neologisms, expressions and idioms while there are some words that are slowly fading away.
Native English speakers can never be right always and non-native speakers can never be wrong always. I have seen many occasions where native and non-native speakers corrected each other's mistakes and grammatical points. That's the beauty of this community.