That's an interesting take on language. If you said many cats were blue, given that cats are never blue I'd certainly feel you were justified in saying many were if 10% of them were.

I tend to interpret many as a relative term, meaning "a high number relative to what you're probably expecting". The dictionary is no help here as it just defines many as "a lot", but that in turn is also a relative term.

I think if you want to argue with this point, a better tack would be to argue that the British PM is underpaid and EU staff are what "normal" compensation should look like. Quibbling over whether 10% of them being paid more qualifies as a lot isn't that worthwhile.