Honest question (directed not necessarily at you specifically) are people seriously bothered by that or is just making conversation and observations?
Because it's pretty damn obvious why no one in the Arrow universe knows about Superman, it's because there wasn't an Supergirl tv show when it started.
Sorry for being captain obvious, but those are just TV shows, they put them in the same "universe" because they think crossovers might be fun down the road.
And yeah, it doesn't make 100% coherent sense, but for fuck's sake, it's a show about a girl who can fly and shoot lasers from her eyes, is this really where you draw line on the suspension of disbelief?
I know some fans demand such things, but I think it just end up hurting those shows, I'd much rather they just try to make the best stories that they can rather than worry all that much about canon and universe bibles. You just end up with a bunch of clunky lampshading, I say fuck it, if you can't explain, you don't need to explain it, just ignore the issue and move along.