In most other universities, professors submit grades and are free to grade their own exams. TAs are there to just get through the stack of exams a little faster than the professor working on that same stack themselves. The TAs don't hold any sacred power in this scenario, they aren't needed at all actually, they just add an extra sets of hands to check or cross out answers on a piece of paper. That's it. There is no bylaw stating your grades must be submitted by a graduate student and no other, but if there were, it is the university that is enforcing the bylaw.

This narrative that grades can only be submitted by the TAs does not seem right, it seems like a narrative designed to put a wedge in the student body by spreading around a fundamental misunderstanding on how grading and graduation occurs at a university.