ZDoom

Yep. Which is precisely how death state lookup is supposed to work.

So... XDeath resolves to Death.Extreme which, since I'm not using exact, falls back to Death?

Both of these are the same internally. When the current damage type system got implemented the hard coded XDeath became a liability so all of its uses, both in definition and lookup get changed to Death.Extreme. The same happens with 'Ice' and 'Burn' and 'Disintegrate' states which also get renamed to their damage type aware equivalents.

Would looking it up with exact=true be able to find both "xdeath" and "death.extreme" if the actor only has the other one explicitly defined?

(and I'm assuming this will be case insensitive)

XDeath is just an alias for Death.Extreme so looking it up with exact=false will return the regular death state. This is by design.

So I'm NOT going crazy! This was why Nashgore's gib check would sometimes weirdly spawn gibs even on actors that don't have an XDeath state.

For some reason, ResolveState and FindState (without exact = true) always succeed when checking for "XDeath" with any actor that's shootable or a missile, even if the actor does not have such a state (e.g.: BossBrain).

[url=https://marisakirisa.me/tmp/gibbables_m.zip]This[/url] sample mod will print to the console the result of various checks for XDeath being available on the actor. Running it on MAP30 makes it succeed for the boss brain, running it on E1M1 shows that the barrels also match up (even though they don't have XDeath). All player-shootable missiles do get matched too, even though none of them have XDeath defined.