OP isn't criticizing theymos decision to ban mixers. He only pointed out the effect of mixer ban in Bitcointalk. But I think that there is no problem anywhere. Theymos decision simply favoured Altcoinstalks forum, and did not take any traffic off from bitcointalk.
BTT just gave Altt some number of participants and did not lose any. It is just about those that has the ability to dualize.
If today theymos makes a rule that it is either you are here or there, but not both, everyone will immediately desert there. But that is not a rule that theymos will make.
Maybe theymos is somewhat happy that another forum got lucky by his decision. Having an alternative forum is fine, incase theymos falls out of love with the US government. So, there's nothing to bother. Where I think will be an issue is in the case of newbies. If the DT bullies here are not lenient on newbies, they might likely run to the other forum where growth is faster and without merits