The effect you are talking about is real, but it's generally small. The cause is called peculiar velocity, and it could introduce uncertainties of a few percent

The effect you are talking about is real, but it's generally small. The cause is called peculiar velocity, and it could introduce uncertainties of a few percent, which is relatively small for cosmological standards. We know how velocities are distributed inside galaxy clusters. It leads to the "finger of god" effect, which makes it appear as if objects would be distributed such that they tend to align along our line of sight. See: https://en.wikipedia.org/wiki/Peculiar_velocity https://en.wikipedia.org/wiki/Peculiar_velocity