Your attitude is misplaced. It's absolutely possible to grow up in a bilingual country without being exposed to all languages.
I admit it would be difficult in Helsinki but I would not be surprised if it could happen in Åland. I know people from Jakobsdal who speak no Finnish.
I don't know how well versed the person you replied to is in Finnish culture but there is no reason for the condescending attitude. I would imagine you could grow up in Greenland without learning Danish, for example. There are definitively Vallonian francophones who never learned Flemish.
Those are all north European examples, don't get me started on Africa or China. It should be obvious that this has nothing to do with intellectual prowess. The question was not stupid at all.