Voldemort knows that Harry understands game theory, and has no incentive to drop his wand if he ends up dead and cannot save everyone anyway. If he orders Harry to drop his wand, Harry might refuse, and then he has to kill him before being able to extract information out of him.

There are two possible answers to this argument.

• 1) If Harry is refusing to give up his wand, this suggests that Harry thinks that with the wand he has a non-0% chance of escape. In that event, getting the wand off him takes priority over questioning.
• 2) Expelliarmus. One of Voldemort's 36 followers must know it, and if not, frankly Voldemort could probably teach them on the spot.

Voldemort knows Harry has knowledge he has not, but this doesn't necessarily mean Harry knows spells or has the magical power required to cast strong enough spells to harm him.

"Power he knows not" strongly implies the ability to do or achieve something, rather than abstract knowledge with no immediate applications.