You have to prove first that Thesis is a derivate work ... it might seem obvious to some, but it's better to ask a real lawyer or at the very least people with adequate paralegal experience ;)

This is a good point, but honestly I doubt this particular provision will be enforced by the courts.

If Thesis is to be held to this provision, it actually requires a contract formation, which means that the Thesis owners must have been aware of this GPL provision and must have intended to enter into a contractual relationship and all the other conditions of contract formations must have existed. This is really questionable. The Thesis owners could say that they were never aware of this provision and they never intended to be bound by it.

Of course if they say that, they will not qualify under the GPL, which may mean they are guilty of copyright infringement (if they do use GPLed code). But it is still possible that they can hold this position and accept their copyright liability (or perhaps try to use other arguments to deny their liability under copyright law). And if they do hold this position and if you have copied the Thesis code, then you are yourself guilty of copyright infringement and have no excuse.

So, I would not rely on section 5 of the GPL in this case.

This is all academic discussion, none of this is legal advice, and if you are actually considering using the Thesis software, please consult your own attorney.