When you start saying 'you want your top defenders to deal with that' you're complicating something that is really simple. Laporte saw where Zinchenko was going, then he saw the guy behind him. In Laporte's eyes they have switched roles, Laporte is now playing left back with a live danger at the back post.
Zin makes two errors, both understandable but both wrong. First he goes central which wasn't necessary (although when it's the second wave of attack as this was it is understandable. An experienced left back wouldn't have done it though). Secondly he doesn't just go to lcb to cover Laporte he ends up in the rcb position because Dias is out of position. There are already two players trying to stop the cross so Dias should be picking up a player.
Also, Laporte wasn't stuck in two minds about which one to defend. When he pointed he made it clear who he was marking. He only tries to get back in behind Lacazette when 1- he sees Zin out of position and 2 - he sees where the ball is going.
You say you wouldn't have blamed him had the guy at the back tapped in but many would, and that sums up the issue of what he was dealing with.
