It's a non-argument, I mean .. Isn't the point of them doing something else with their PHP knowledge, and no longer doing anything for Jelsoft .. one of the motivations behing XenForo? That's how it looks to me in my opinion at least.
Why on earth would they even want to use source or features from vBulletin (4) which obviously are not working for it? The whole point (in my observation as a customer of both companies) was to do something apparently that was more modern, from a perspective of being a player in the bbs market, as well as from the needs of customers.
vBulletin 3 and 4 are html4, xhtml1, with css2, xenforo is html5/css3. They used YUI, xenforo used jQuery. The list goes on. The source code is (so it seems) started around the zend framework, I have not seen any evidence of zend or a full fledged framework on vBulletin 4.
The claim about trade secrets, like "an improved more automated upgrade or installation system" <--- isn't that public knowledge, i mean .. everybody wants to improve theirs, and have it automated. Did vBulletin see what Wordpress did? They basically have a trademark for a upload, press go, and get your password .. installation. Who's to say that's not what XF was inspired by?
It's grasping at straws, throwing stuff at a wall and see what sticks. Corporate bullying, financial draining of competitors.
I think we've gone through this before though ..