It’s very hard for us to sayy that Rashi’s Mekor had to any specific place. There are many Midrashim that were around then that are not not around anymore. If Targum Yonasan was composed or compiled during the Rishonim period, the author drew from these same Medrashim. We can’t bring any proof.
The Ramban, however, does quote the Nusach Yerushalmi of this Targum. There are those who suggest that the Targum is Actually Targum Yerushalmi, abbriviated as TY, and mistaken for Targum Yonasan. Or perhaps, it was from some other, later Yonasan. There were many Targumin in earlier times. There’s Targum Reb Yosef, who some say might be out Targum Yonasan (TY), but I heard this was disproven by contrasting it with his Shittos ‐ although it is possible that he included other Shitos.
This Targum does have something in common with Targum Yonasan on Nach. They both veer from Pshat often enough to explain a Nimshal or some Medrash. Although, this Targum seems to do this more than the TY on Nach.
The fact that the Gemara says that Unkelos did the Targum on the Torah and Yonasan ben Uziel was Metargem the Nach doesn’t necessarily show that he didn’t author a parallel, alternative, non-essential Targum.
Tosafos writes that when the Gemara says we must say the Targum even for Ateres Vedivon, it means that although Targum Unkelos was not Metargem these words, we should be Metargem. If Unkelos wasn’t Metargem them but there is another Targum that is Metargem, then we should use that other Targum to be Metargem these words. It happens to be,