The actual prerequisite is not the ability to cast level 9 spells, but being an epic sorcerer or wizard, or having at least 15 pale master levels. Furthermore, this feat can only be chosen when gaining a level in the qualifying class.
LORE's
current version has the vanilla description about only needing the ability to cast level 9 spells, so it should probably be either edited to reflect the correct requirements or added in a note.
Edit - Sorry, I was a little hasty with the detail. The current note on the page is:
The twenty-one (21) caster levels must be gained in a base class (Bard, Cleric, Druid, Sorcerer, or Wizard), with the Pale Master Prestige Class being the one exception.
Druid and Cleric need to be removed. Bards can still take it. (Sorry.)