I think Griszt recognized Thaniel's name because Thaniel was well on his way to becoming famous before he gave music up to care for Annabel et al. I mean, this is someone who can play Mozart and Handel from memory after four years of zero practice, and we're constantly told that he shut down entire wings of his mind after he stopped playing. It makes sense to me that refusing to think about what he could have been (i.e. a renowned concert pianist) would be part of the furniture in them.