It is called a god stat block. In my campaign, I had a god create a tidal wave 100 feet high that was hundreds of miles long, and washed some 20 miles inland. There is no spell, no effect, in the game that can do that. But I can handwave it as a DM, because the DM is the god of gods.
But to your point, Venca's actual ability scores would be miles higher, prof added to all saves, a whole whack of features like "if your char is below 12th level, and Vecna points his finger at you, you are dead. No questions asked, no saves,, nothing. And dead dead. Like soul eaten dead."