The Prydain books actually specifically don’t use the secret royalty trope tho? Taran never finds out who his bio family is.

He does end up on the throne, though, and Dallben and the rest all generally assumed that's where he was going even if they weren't 100% sure. That's the main reason I put it at least partially under that same umbrella. I completely see the stance that it doesn't, however.