And if you read a little farther into the book... you'd realize that Dumbledore said, "He hoped that I would sacrifice you in the hopes of killing him." He never said that he could kill him.
Also in the first book Dumbledore said that the Dark Lord had powers that he will never have.
Dumbledore did say that voldemort created spells that he never would create. and also vice versa.
Voldemort said that Dumbledore was the only one he feared because of his power, wisdom and because he had the ELDER wand.
The elder wand gives its wizard or witch the ability to always end up victorious in any duel.
Dumbledore easily could defeat voldemort if he wanted to. but chose not to. he had the elder wand.
The horcruxes were a measure of reassurance for voldemort. If his current body is destroyed by the killing curse he could transfer the rest of his soul into an object which already has a 7th of his soul left in it.
I said that dumbledore can kill voldemort if he wanted to.
Dumbledore can win tyhe duel between voldemort, kill him using the killing curse. Voldemort would transfer his soul into another object or person. Then dumbledore would kill him again.
and so on.
If volemort had one life, from the first moment dumbledore would kill him, he'd be out of their world, but since voldemort has 7 lives he would have to be killed 7 times. Dumbledore still can do that.
Its just a matter of quantity. Not a matter of who is stronger.
Everyone knows as shown through the hp series that dumbledore has the CAPABILITY to kill voldemort. Its lather rinse repeat from then on.