I know that. However, his existence doesn't preclude SG being part of the Flarrow-verse in any way.
Except no one
generally knew about powered people in the Flash/Arrow universe before the STAR Labs accident, and even afterwards the public seemed to have a hard time believing in the powered guys in Central City at first, at least based on how we saw the police react. While we've had one powered villlain to prove that people have gotten powers in other ways, I think having Superman as a well established fact would have been mentioned, and it would have lead to the Flash team, and especially the Arrow team, reacting differently to people with powers normal people don't have, especially ones not created by the STAR Labs accident.
I can't see any way that Superman could have been established in the Pre-Flash Arrowverse, and The Flash show has pretty well shown that there was no
generally known super people before Flash, at least not at the level that Superman seems to be in SG.