iirc, it was K and FrankT
that chimed in, perhaps the last time they did so together, on the old TO board
with the Polymorph update circa then.
Spellcasting "was" either ex or su or sp , even if undefined.
Hence, polymorph could pick up those abilities too.
That's where the polymorphing to get the monsters spellcasting ability started.
Minor detail ... a psionic power produces a (ps) effect.
It's the effect that's equal to a (sp) effect, totally equal.
The ability that produces the effect, is not.
What exactly that ability is, has never been determined.
Then it flips back onto what K / FrankT posted on TO.
You can get it, even if you don't know exactly what it is.
Cheesy yes. RAW yes.
So ... you don't know what kind of ability spellcasting is,
except that it is (ex) or (su) or (sp) ... you still don't know.
And you still can get it.