Maverick Windesburg was well aware of how powerful it was now. Even, due to the rules of the game world, his true power was suppressed.
Otherwise, his true power was just below the weakest Outer God as he was someone who had achieved immortality.
All of his dark magic experiments were held in order to have a glimpse of immortality. It just so happened that his experiments weren't all in vain and he finally achieved it a hundred years ago.
Furthermore, Maverick was a Special NPC so he knew some secrets that aren't known to the average NPC. For example, he recognized Weiss as the God of Games Aspro.
"To be honest, I'm quite confident that I'm stronger than Aspro if it's the power in the real world. My true power can already destroy multiple worlds and if I were to prepare well enough, even destroying a miniature galaxy is possible."