IF they use the 5e rules flying forms are not available to level 8 for either type of druid
They don't. Dire Raven is available at level 4 for Circle of Moon.
This might be for testing purposes. They could change it to all druids, level 8 once the game actually has a level 8!
Level 4 should give swimming, but since the game don't have swimming, I can't see that happening. Could add a token crocodile though. Technically, the polar bear has a swim speed and
CR 2, so shouldn't be available until level 6, but the BG3 version is weaker than the PnP version.
As per PnP, land druids should get level 2 moon druid forms at level 8. Moon druids should get tougher forms at levels 6 and 9.