All civs can only have 1 bias. Any river bias civ has nothing to determine what terrain they are in. So Songhai, who has a river bias, has no preference for plains vs desert vs tundra. Etc. there are also lots of civs with no start bias at all that we allow to free-float, like Sweden or Russia.
IMO it’s good to remove start bias from Siam because I think the forest bias is essentializing. Not only does freeing up start placements benefit the whole game, I would argue it centres the culture onto the people, distinct and independent of environment. Like the Thai people would still be Thai if they weren’t in a jungle. I think the forest biases, desert biases etc sometimes hew too far into treating some cultures like they’re elves or something.