The Onion is, literally, fake news regardless of whether one finds them funny or not. The question is whether people think it's true not whether they think it's funny and that's the only decision being made by a fact-check.
Different standards were not applied. Where are you getting that from?
Forum users posted the article from BB asking for it to be debunked. They haven't posted a request for articles from the Onion and that's likely because the Onion is well-known enough for people to know it's satire.
The Onion is, literally, fake news regardless of whether one finds them funny or not. The question is whether people think it's true not whether they think it's funny and that's the only decision being made by a fact-check.