Wouldn't it be scarier if he didn't hear a knock? Just knowing that other intelligent beings are out there makes it kind of a Cosy Catastrophe.
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Not hearing a knock A. means no story, and B. isn't scary, it's depressing.
Hearing the knock, when you know for certain there should not be any knocking going on, evokes mystery, and there's nothing humans fear more than what they don't understand or expect. There is something there when there should be nothing there.
And knowing that other intelligent beings are out there doesn't mean they're friendly intelligent beings.
Whether they like me or not, I wish them well. Maybe they'll do better than we did.
Also, horror isn't just surprise, it's suspense. If you set up a premise (that everybody's dead) and then suddenly negate it, there's a bit of a shock, but no build-up. Suspense comes from knowing in advance that something horrible is going to happen, but not exactly how or when.
In this case, the suspense is in not knowing what is going to happen.
Heck, depending on how you look at it, nothing, in fact, may be happening. Perhaps the knock is all in his head, and this is the first step of him going mad.
If we take the narrator at face value, the character is the last man on Earth. That means whatever's knocking is not human. Ergo, the premise isn't negated; all the humans are dead, but there's something out there.
A far more suspenseful version of this story is this. The last man on Earth sat alone in a room. There was a lock on the door. It makes the reader far more curious. What is the need for the lock? Is the man insane? What is he afraid of? Comparatively, the original story seems like a one way street, while this is an intersection teeming with possibilites.