I immediately think of how Eldritch Knight and Arcane Trickster both work. The base Fighter and Rogue chasses don’t have spellcasting (nor do most of the subclasses), and there’s no Fighter spell list or Rogue spell list. Yet those two subclasses gain a Spellcasting ability and draw from another spell list (in this case, the Wizard’s). Mearls seems to be describing subclasses that gain a “Spellcasting” ability that draws from an out-of-class spell list, in this case being a Psionics spell list.
I put Spellcasting in quotes because I imagine that Psionics could easily be like Pact Magic, an ability that operates *like* Spellcasting but is separate from it terms of slots and levels. Note that, even though Warlocks don’t “cast spells” through the same mechanism that other classes do (and thus interoperable their slots when multiclassing), they use the same mechanics and terms when defining spell lists and spell levels.
I see Psionics being done easily in this sort of way, without having to create new complicated terms and mechanics — treating Psionics as a magic mechanic on the same lines as Pact Magic and building psionic subclasses similar to how the non-Wizard subclasses which draw from that spell list operate (the only difference being the lack of a “Psionic” class that houses that list, the way the Wizard class houses the list from which Eldritch Knight and Arcane Trickster draw from).
