You're assuming that he
should rightfully be considered a nunh. I don't believe that he ever was, and if he could have been at one point then he isn't any more.
For now he is G'raha Tia and he introduced himself as such in the ending cutscene. Rammbroes is still calling him that as well, and I'm sure Urianger wouldn't be so informal as to drop the title even if everyone else did.
But the problem isn't with what the characters are choosing to do, it's with the information in the first place. (I'm on the verge of taking this to the localisation forum because they don't seem to be able to keep their own lore straight on this subject.) Y'shtola's explanation simply does not line up with the official
Naming Conventions - tia doesn't specifically denote anything, but is the default state of not being a nunh, even for someone who previously was a nunh but was ousted by a challenger. And G'raha is definitely not a nunh.
He could maybe theoretically have claimed the title
while he was Exarch, not that it would count for anything in the First, but it would be bending the definition and in any case he'd essentially abandoned his name and identity as a Seeker at that point - he couldn't use it openly because his plan relied on it remaining unknown, and his expected path led him to a place where he'd never have a reason to take it up again.
Now, as G'raha himself says, he is no longer leader of the Crystarium and holds no territory or power. He has not claimed a tribe. He
is currently a tia and has no reason to be addressed otherwise, unless Y'shtola's dialogue is an outright attempt to rewrite the lore. She should be correcting Alphinaud on his misunderstanding, not repeating it.