The rule is, if they're in legal guarding position and receive significant enough contact to displace them from said legal guarding position, it's an offensive foul.
It's a stupid rule, and this was a textbook example of why it's stupid. He didn't establish legal guarding position until it was too late for THT to veer away. Just a dumbass rule.
Yeah, that’s just completely idiotic. By definition any offensive push off to create space should be an offensive foul then.