the new net Q needs to look at a essentially so that is a true statement so if my Q if my Q is so powerful that it could just represent the whole marginal distribution of B so let's say let's call if we have Q of P given a that is equal to Q of B for all a and B pair then your statement would be true basically you can just like approximate the marginal distribution without even doing any meaningful work so that's why like in practice people would have a fairly restrictive mapping so like that's why like in most of the works