To be fair, I wondered how the angle would make it stronger too :+ I've dented all kinds of rim in all kinds of ways
I guessed he meant pinch flat kinda dents, that the force applied straight/square on to the edge would push the rim in the direction its leaning. If it wasn't leaning, less inward dent.
* but then I re-read this topic and it changes form one to the other
It sooo depends on circumstance though imo.