Firstly, you are making the assumption that it can't be a death in the apartment, else HCW would know that. He may not know where and when it happened. If their evidence is just proof of CB killing MM and not much else, that doesn’t specifically mean that the death must have happened after the apartment.
Secondly, he is answering a question specifically about how confidently they can place CB in PDL on the 3rd. He only uses the proof of death at CB's hands argument as a way to re-inforce the probability of CB therefore being there on the 3rd. But, as a caveat he admits this particular evidence does not specifically identify the date the death occurred. So for example, if the evidence they have is a port mortem photo of MM, obtained from a source attributable to CB, then that would fit. It gives them confidence that CB is responsible and was there in PDL on the 3rd, but the evidence itself gives no accurate indication of when the death had occurred. Even if the photo EXIF data showed a date of the 4th, would it realistically be possible to determine when the death had occurred from that photo alone?
So no, he hasn't actually said MM died after leaving the apartment. That's just how you're interpreting it. And just to be clear, I'm not saying that your assumption can't be correct. It might well be, but the point is, HCW hasn't explicitly said that MM died after being abducted from the apartment. And the other comments that might suggest that, can be interpreted in very many ways.
