The Gemara could be referring to the main Targum. There was more than one Targum. Reb Yosef also made a Targum. I think I’ve heard of a theory that the other Targum in our Chumash is really Targum Yerushalmi, and was abbreviated to TY, which was mistakenly unabbreviated to Targum Yonason. It’s a hard pill to swallow, but if you like it feel free to swallow it.
