XP Rules for Monsters With Innate Spellcasting

[Zanos]

D&D is a roleplaying game, not a simulation. The rules are an abstraction for the players to interact with the game, not how the game world actually works (unless you're playing in a OOTS-like campaign at least).
Joe Commoner can't become level 8 in a few weeks by going out to kill a CR 1 animal every day (a PC could) and NPC crafters don't need to go murderhoboing to collect the souls of their enemies as crafting fuel. Not by default at least.

They're an abstraction, but they're not without grounding in phenomena observable in character. It's recognized that crafting magic items or casting XP component spells drains some sort or vital essence, and that attempts to do so will fail if you don't have enough. NPCs recognize that spells like Wish or Permanency are limited by forces other than spell slots.

That said, SLAs don't have any components, including XP. Supernatural abilities also probably don't. If a creature is casting actual XP spells you need to make up some nonsense for how much it can spend; the dragon example is probably a reasonable guideline for a very powerful and ancient creature.