A theory doesn't prove anything. It suggests an explanation that must then be proven.
Ever heard of Halon's razor ? "Never attribute to malice what that which is adequately explained by stupidity/incompetence"

It is much more likely that the devs just don't know what to do with dark knight, or didn't have time to work on it, than for them to purposefully sabotage their own work to piss off someone who isn't even working with them anymore.

"Oh boy, my last colleague was a pain for (insert unknown reason), now that they have left I will make sure that the part of our group project they worked on becomes terrible !"

Here is another theory, that explains all:
The devs' dogs ate their homework with the dark knight updates in it, and they had to redo something from scratch in the last minute because they have terrible memory and couldn't remember what they did before.

It explains it all ! You don't believe me ? Prove me it's wrong then. Prove me that they didn't lose the actual EW Dark knight updates and forgot all about their competence and skills until the last minute.