Polytheistic religions just seeing multiple advanced beings and calling them all gods because thats how they interpret them, and the one God most believe in May well just be one of them.
In fact, Paul went to Rome, where they had a monument to the “unknown god”, just to hedge their bets to make sure they didn’t leave any god(s) out at risk of angering them, and he told them that that god is actually God Himself.
JEDIT: Wait, I read your post wrong. You’re saying the ancient polytheistic gods might have existed and if so been like the Stargate Goa’uld?