HGD can block anything, if the Solar is able to effectively employ it. Explicit examples given include sorcerous curses and exploding supervolcanoes, neither of which are decisive attacks.
Not quite. See, HGD has three different modes of operation:
Spend 4 motes plus initiative to remove successes from a decisive attack. This one only works on decisive attacks.
Spend 4 motes to block an otherwise-unblockable attack. This doesn't let you block anything that isn't an attack, it only removes the "unblockable" trait. (Because of the Decisive-only tag, it still only works on decisive attacks, but that's probably an oversight; it seems like it should work on withering attacks, too). This version of the charm can't block anything that doesn't have an attack roll, because in the absence of a "this always works" or similar, it's mechanically nonsensical. It explicitly works on "the burning curses of Kimbery", but that just means that they're unblockable attacks.
Spend 4 motes 1 willpower to automatically block something that does uncountable damage. This is the version that always works and explicitly works on erupting volcanoes, but it explicitly
only works on uncountable damage.
Only one of these options works on non-attack damage, and it doesn't work on countable damage.
AST explicitly protects against falls from a great height. Are Solars now cats, who will die if they don't fall far enough?
Reading the charm as written, that's the only possible interpretation. It protects against uncountable damage, including falls from a great height. If you look at the falling damage, it lists countable amounts of damage. How can we resolve this contradiction? Well, the only thing that makes sense is that for a really
really long fall, the damage becomes uncountable, at which point AST becomes applicable.
Yeah, the athletics charms are better for keeping your balance, but this one has the advantage that it's permanent.
More importantly, Graceful Crane Stance allows you to balance on things that would otherwise snap under your weight, or otherwise be completely impossible to balance on.