Nitpicker, it is actually Nusach Yerushalmi of the same Targum. That’s why there are only few differences. Many of the differences are merely linguistic.
The Ramban rarely mentions it. So, if other Rishonim refer to it one time less than the Ramban, they won’t mention it at all.
It makes a lot of sense that it was written after many other Midrashim. He brings many Midrashim and is not the source for Midrashim. Maybe a Gaon wrote it in his spirit, like the Raya Mehimna, which according to the Kamarna was written by a Gaon named Shalom. He wasn’t sure if this is the famous Mar Shalom Gaon.