The reason his foot went past you is because he chose the wrong kick. You would have moved into a roundhouse.
This statement is like telling someone who ducks a jab that the reason he didn't get hit was because the guy throwing the jab threw the wrong punch and should have thrown an uppercut.
The benefit of attacking at angles is that it makes your opponent attack where you were instead of where you are going to be. If he was in a position to do a roundhouse then I would have factored it and would have done something other than a back fist as a way to address a possible roundhouse. Had he not punched which would have resulted in me doing an empty trap then I would have not continued with the backfist because his right hand would still be free and not controlled.
I actually have a video where I do an empty trap because my opponent didn't throw the punch combo that I thought he would. I abandoned my original plans of attack with an expectation that he would do a round house kick. Here are clips from that video
Here you can see that I do an empty trap because my opponent didn't move in as I thought he would. Notice that he's in a good position to launch a round house and being that he used to do TKD. I take his roundhouse serious, he's our strongest kicker for the roundhouse kick and could easily break ribs. I'm saying this so you understand that he knows how to throw a good roundhouse. At this point I know I'm in danger.
There are 2 rules that I have for a roundhouse kick. 1. The power comes at the end of this type of kick. 2. When I see it or think it's coming then move forward so that I don't get hit with 100% of the force of the kick. This is what I did in this video:
1. I moved forward and at the same time I grab his guard with my right hand to prevent the right hand from being able to hit me. If I get hit, the the kick won't land at full force.
2. If someone kicks then it means that they are rooted on one leg. If I can disrupt or destroy the root then it will reduce the power of at a roundhouse kick. This is what you are seeing in the picture below. The reason he's leaning back is because I'm too close and he's trying to adjust. Take a look at the location of my left foot in the picture above and compare it to where it is located now.
3. My left hand is free to deal with that roundhouse kick coming in. I know that I will be ok with blocking it with my left hand because I'm not on the power end of the kick
In this picture you can see that my foot destroys his root. This is the disruption that a "tap" to his root causes. He would have gone airborne if I kicked him with my full force. Take notice of how my left arm is blocking the kick coming in. Take a look at how well I'm rooted. The only reason this happened is because my initial trap was a big fail.