Prepared-but-not-PHB-Known is possible via the Sha'ir. I'm not suggesting that Absorption should be interpreted with that knowledge though, as Sha'ir is a more peripheral class. When considering absorption previously, I had defaulted into your interpretation. Your interpretation also happens to be a more minimal affordance than the alternative.
The definitions we've gone through have known spells =
- Spells that a spontaneous caster knows intrinsically (as per Absorption)
- Type 1 + Arcane Spellcaster + has learned + can prepare (as per PHB glossary)
- Type 1 or prepared spells (as per Divine Metamagic)
- The entire list for Cleric (as per Eye of Horus-Re)
- Type 2 or the entire list for Druid/Cleric (as per Place Magic)
- Accessible spell (as per Scribe Scroll)
All of this makes it clear there are many more gradations of 'know' than the PHB definition.
Rashemi Spirit Magic still seems ambiguous however. It's at least type 2 when interacting with arcane casters. The minimal affordance with divine casters is type 3 but Place Magic provides precedence for type 5.