Checkpoints would require a TW attack to start after the CP, and is a manual intervention. The devs probably did it to help restore calm more than anything else.
I wasn't disagreeing with you in my prior post (hadn't even read your post yet), because I meant the TW attack could intermix the legitimate and attack chains from the CPs forward.
I like disagreements. It often means I am about to learn something, which generally makes my day.
I think you have the concept accurately. Any miners with current code (that include the CP) will not honor a chain having a block at the height of the CP that doesn't match the CP hash, but from there forward, it is open season once again.