How do we actually know that Orlanth and Humakt existed virtually as they
are "now" known before the Dawn? How much of "true" pre-Godlearner
mythology still exists? The Godlearners didn't need to necessarily rule
the Orlanthi to take over their myths--I would speculate that this was
one of the things that their deep space missions discovered.
Indeed, isn't there a statement somewhere to the effect that the
Godlearners were surprised to discover that deities they created (and
Orlanth is cited as one such deity) were independent to the point of
ultimately fighting against the Godlearners?