Considering that both the main Vib translation as well as the "official" Viz translation say the same thing (essentially), I'd say it certainly isn't a mistranslation, and especially not an accidental one. Yeah, we don't see the specific kanji in the raws, but that doesn't mean that the actual translations are debunked. They aren't just looking at kanji and searching for its definition, they understand how to read kanji, which is a complex written language, and we (or at least most of us) simply don't, the opinion of the translators is naturally more validated, although I'd like to get an actual response from one of them about the subject.
Either way, it's not that big a deal to be honest. It's just a contradictory throwaway hyperbole statement and shouldn't be taken seriously. The only annoying thing about it is the amount of Saitama fanboys that are gonna bring it up thinking it makes him unbeatable, but that isn't anything new.