The issue is not as to whether an attack hits at all. Rather, it's to dealing super-effective damage to Spiritomb and Sableye. You are correct with your statement regarding Fire Fang, though.
Originally Posted by Smogon
All Pokémon on the user's team that are not fainted and do not have a status condition do a 10 power
typeless attack on the target. Beat Up uses base Attack and base Defense for these calculations; EVs, IVs, and Natures are irrelevant.
Beat Up hits through Wonder Guard, even if the Pokémon isn't weak to Dark. If the user targets its ally in 2v2, this results in the ally beating itself up if it doesn't have a status condition.
See? Beat Up is typeless, and hits through Wonder Guard regardless of the type.
The fire fang glitch only works in D/P.
I know.