It does not actually work like this. I am sure Thesis does include at least some original (i.e., not previously GPLed) work. Thus, the person that created that original work is the proper copyright holder. Thus, if you copy Thesis you will be guilty of copyright infringement, and the fact that it does include some GPL code won't help you.

As the matter now stands, if it is true that they use GPLed code, that means that whoever distributes Thesis is guilty of copyright infringement, which means that the owner of the original GPLed code can sue Thesis for lots of money. But that does not mean that anyone can copy Thesis.

Put in other words, the viral effect of the GPL is not automatic. If you use GPLed code in your code it does not mean that your code is automatically GPLed. It means that you have a choice to (i) voluntarily GPL your code, (ii) get rid of the GPLed code from your code or (iii) be guilty of copyright infringement and face the consequences.

This is intended as an intellectual discussion of the matter and is not legal advice.