Nah, the knocking was probably just to make sure nobody was home. Pretty common ploy.
This. Had you answered the door, I imagine the perp would have had one of two responses:
1) Disengage- offer to sell magazines or some other junk, expecting you to refuse and they'd leave.
2) Engage more violently- come up with some story to enter your home (hey, my car broke down, can I call my cousin?), whereupon he would draw his weapon and it would become a full blown home invasion, not just burglary.
Since the guy was armed, I'd expect he was thinking more option 2. Somebody looking to burglarize and arms himself for that is NOT likely to back down from a confrontation if they think they can come out on top.