I consider bug using any kind of behaviour that gives you an advantage by exploiting something. For instance getting stuck in the wall makes the difference in your case, from killing that boss or not killing that boss. Thats an exploit and is bug using. Your other example is simply ridicolous and doesn't make any sense. Not going to ban you if you use that, but not going to help you either if you get screwed over or you box drops inside the wall and out of your reach.
It is unfixable. You used the poly after you were knocked back. When you use a poly or mount on horse you are refreshing your position, which was set off by the knockback attack, so you found yourself inside the wall, well more inside than you wanted to be.
Oh my freaking God. You still don't understand what happened. No you do not want to be stuck inside the wall when killing azrael.
You go to the wall so that his knockback does simply knock you against the wall. After you kill him you can run around freely, because you are simply not inside the freaking wall.
Did you think that the intention is to get stuck inside the wall? No, simply no. It's similar to hitting a boss against the wall with your horse, so you keep hitting him without knockback, just reversed. I would hardly call this bugusing.