The buff description also specifically states "all mundane projectiles", although whether a cloud of magically-conjured metal frisbees counts as 'mundane' or 'magical' is another thing.
But yeah, AoE things go right through.
EDIT: Okay, not sure where I got the mundane projectiles thing, because it doesn't show up on the normal air shield spell description or buff description... I could've sworn there was at least some variant of the effect that specified it was for defense against mundane projectiles.
Anyways, did some testing with Nazca and Twiceborn, because I got curious. Turns out, TB takes priority over mummification, so if a coya or hurin priest has cast TB and dies, they will return as a wight mage and will NOT come back as a mummy.
However, it has no effect on the ritual sacrifice of a coya if an inca dies and is to be mummified. TB gets completely bypassed, and the coya dies and is made part of the mummy bundle as usual.
What I was really testing, however, was reanimation. Unfortunately, incas cannot naturally spawn with death magic, which is a shame because they're the only living H3 priest Nazca has... But if you empower an inca, hand him a skull staff to cast Twiceborn, then send him off to get killed (preferably after returning the staff to the lab...), the resulting wight mage will be able to call supayas, same as the royal mallqui. The difference being that the wight mage costs no upkeep.
...now, mind, that's 60 death gems put into making the damn thing, discounting the staff. For that price you could cast "summon supayas" 6 times, resulting in 30-42 supayas (if I'm remembering correctly and the headdress adds +2 to summon spells). It'd take a reanimator 10 turns with a headdress to summon 30 supayas, and 14 turns to get 42.
So if you're looking at a long-term investment, I suppose it's technically a possibility...
I also learned some other interesting stuff, like how the "reanimate archers" spell (banefire bows, whoop whoop) is affected by reanimator bonuses. This means that Manco-Ocllo, who has a 4/4 bonus naturally, when wielding an amulet of the dead, will raise 16 archers per casting. Just a nifty little tidbit.
Also, geoglyphs instantly raises the magic scale of the province it's cast in by 2 points, which is pretty huge. This lasts for the entire duration of the spell, which I *believe* is 3 turns +1/extra pearl spent? Not quite sure.
EDIT2: In case anyone was wondering, putting a fever fetish and a disease grinder on the same unit will let both items work simultaneously. +2 fire gems and +1 death gems per turn.