Banning based on this doesn't seems to make a whole lot of sense.

First, as far as I can tell, a node that do not have the assumevalid block will simply return the 2000 first block it knows, so it's not going to trip here, but worse:
1/ You'll ban nodes doing IBD.
2/ You won't have the information to know if the node is a BCH node or note based on the 2000 first blocks. It could be following the BTC or the BSV chain or simply not having finished IBD.

Lastly, assumevalid means that we assume the block is valid (I know, right ?!). The block being valid doesn't necessarily imply that no other block is valid. But we assume this specific one is.

What you actually want to compare against are checkpoints.

Why is it that you want to continue to process messages from that node AFTER you decided to ban it?

Clearly, you did not gpt a version message here, but headers.

There is no modification in that header, so it is unlikely that new includes are required.

This is not a message that exists.

The lack of if statement bellow makes it very clear that this is NOT the size of the message. the size of the message is also very clearly not 0 or 1.

Presumably you'd want to test that the node is not banned.

This is not really testing what we need/want to be testing. We want to make sure we keep nodes that follow a certain chain and reject these that do not. There are intermediary steps here, but it is clear that this test is not helping because the does the completely wrong thing, and the test effectively test the corresponding wrong behavior.