I can't say I blame him for putting up a fight, since it's his bread-and-butter and GPLing it would cut his financial knees out from under him, but it's not even a debate at this point whether or not GPL code is included in Thesis. This is a fight that he can't win.
(I'm not well enough versed in this particular debate to weigh in, but there is an answer to that question, in general, and it hinges on specific details of one's implementation of "creating something.")
In the end, it's the laws of the land that you're in that matters. As far as I know, I don't believe that these arguments on what is a derivative work are anything more than opinion. Settled cases are not equivalent to legal precedent.
The best thing that could happen for everyone involved is that it goes to trial and ends with a judgement, and then all this posturing and pontificating will hopefully come to a merciful end.