This is probably a place where Q’s original lesson on the curse in MOR (the one where he keeps saying, “One killing curse will bring it down!”) might have been usefully supplemented by it being mentioned that it’s the only unblockable spell—as opposed to just saying that nobody can block it.
However, there’s plenty of places in MOR where shock and awe are expressed over Harry’s surviving a (presumed) direct hit from the killing curse. IOW, it’s shown in MOR that the killing curse is unblockable and that this is an important fact about it. But perhaps for this particular plot point, it isn’t emphasized enough that it’s the only fighting spell that can’t be blocked by something.
This is probably a place where Q’s original lesson on the curse in MOR (the one where he keeps saying, “One killing curse will bring it down!”) might have been usefully supplemented by it being mentioned that it’s the only unblockable spell—as opposed to just saying that nobody can block it.
However, there’s plenty of places in MOR where shock and awe are expressed over Harry’s surviving a (presumed) direct hit from the killing curse. IOW, it’s shown in MOR that the killing curse is unblockable and that this is an important fact about it. But perhaps for this particular plot point, it isn’t emphasized enough that it’s the only fighting spell that can’t be blocked by something.