It's not unreasonable in a partnership like BMW-Toyota that one aspect such as transmission options affect both cars. If Toyota doesn't like the 6MT BMW offers, that means the Z4 doesn't get one either.You can like the Toyota brand while remaining realistic about its engineering and marketing decisions.
Saying that BMW could offer something but Toyota decided it was below their standards is not being realistic.
A new 6MT is most likely going to come from a partnership since they're pulling from BMW supply lines and so it's also reasonable to assume that if the i6 Supra gets a 6MT, so does Z4. Toyota isn't reasonably going to make the manual themselves since that's going to disrupt production lines.