Firstly, in a summary, it says: "License of Avast is free, but you may wanna use this method if you don't have an internet connection or don't wanna deal with getting license."
I think they state this method for the home version. And they only change system clock. Possibly this must be also applicable for the pro version. Yes, it is unethical to do this on the pro (maybe some of us will defend that it is unethical for the home version too); but there is no illegal behavior at all, actually.