I dunno... I was a child when I noticed that toilet paper is different from other paper, in that it degrades pretty rapidly when wet. Feces also obviously doesn't hold its shape very well. That seems fairly straightforward and 'logical', and my instinct wouldn't be to flush anything that doesn't have a decent chance of breaking up in water. Again, most kids would know that latex and rubbery things don't break up in water.
I've never looked up the way the plumbing system works and what it can handle, and was never taught much about it, but it was pretty easy to come to the subconscious conclusion that flushing is (generally speaking) for liquids, poop, and TP.
I will say that I wouldn't expect anyone to know that (as described in the article) a product explicitly marketed as "flushable" wouldn't be good for the water system. But that's a separate topic IMO: a systemic failure in providing inaccurate information vs making assumptions that don't err on the side of caution.