What am I missing here?

Not as much as I'm missing. They were in the final stage of an important war(game), so climbing icy walls with magic technological help seems to me a minor discomfort, just part of the game. They all had Feather Fall potions, so no-one was in any danger, whatever the lizard brain thinks of looking down from a long way up. Hermione even told Draco to drop her (according to the possibly unreliable girls' chatter afterwards). Harry had no way of knowing exactly how the rooftop chase would play out, although I would guess that he secretly practiced beforehand to get an advantage.

So, what is there for Harry to apologise for, and in such an extreme manner? I was expecting something else to be revealed in 42, but apparently not.

I know nothing about canon HP, but I don't think that matters here.