That's a ridiculous claim
Edit: It may be that this is only an issue inside of Firefox extensions (addons). In which case, maybe the point is moot. See: https://github.com/mozilla/addons-linter/blob/master/docs/th...
I am not sure that being asked not to disclose is a good enough reason without further justification; in fact it seems like a poor reason to me. Mozilla is in my view kind of a shepherd for internet users and I'd hope they'd fall more on the side of "let's not let our users get owned unnecessarily" than that of "let's sit on this vulnerability just because the disclosing party asked us to."
It was hyperbole. I personally think that the language is clear too. Here's a good post explaining the usage of the phrase "For all I know" since apparently HN isn't familiar with the idiom https://english.stackexchange.com/questions/92207/what-is-th...