A sigil with “enemies are immune to death damage” prevents death creatures from interacting with the enemy in any way. Spells, attacks, mana reduction, everything.
A sigil with “enemies are immune to spell damage” prevents all spell effects, including debuffs, mind trickery etc.
Creature class is used to determine what is prevented rather than spell gem class.
Also, the prevention is abrupt and without a notice. It would be good to add a message such as “the sigil’s magic protects enemies from your spells”
Possibly semantics on the first two and the descriptions just need to be updated.