> since when do you have to be vetted/credentialed to participate in a discussion here

Since they're using their credentials to try and lend credulity to their argument. Specifically, it's an "appeal to [their] authority" on the subject, but they don't have that requisite 'authority'.

Since when is providing some background about your experience considered a negative? You should try to be a bit more generous and not assume, on almost non-existent evidence, that someone is being disingenuous.