Just define intelligence as "ability to do a task". The human brain isn't magical, it can simulated, so we know for certain that a powerful enough AI can be at

Just define intelligence as "ability to do a task". The human brain isn't magical, it can simulated, so we know for certain that a powerful enough AI can be at least as skilled at any given task as the most skilled human who has ever lived. So for any given person, consider how well the most effective persuader out of 8 billion people would be at persuading them. Maybe they can't always persuade them of anything, but pretty close. This is the lower bound for an AI's maximum theoretical abilities, and it's already enough to rule the world.