Maybe you're confused by the way you are interpreting the word deny. It's more like how could you disprove the universe/world wasn't created a mere 5 minutes ago, never mind 3 years ago. There's nothing we can do to disprove this, and though it's not explicitly stated in this text, the thing they are getting at is that all of our memories could all be false so they aren't sufficient evidence to prove the universe has existed for X amount of time.

