I agree these are possibilities. But the Chinese sources for the phony PCGS holders aren't generally doing "one-offs", they're churning these out as an industrial product, like ersatz Nike shoes. So I'd expect in the vast majority of cases for the encased coins to be fake, too. Now, even if the coin within was genuine, there may yet be a detectable weight difference on account of the nature of the material the counterfeit holders are made of as compared to the authentic PCGS ones.
I agree...with the current Chinese production this does make more sense. That said, I have heard of problem coins being put in problem-free slabs as a scam too. However, I think those are more of the case of an authentic slab being tampered with. The good coin removed and a problem coin inserted.