I don’t think the idea is that the Yerushalmi says if someone comesto you in a dream saying you should convert, dig him up and find a warp and weft. The Gra probably had a Diyuk from some Yerusalmi, that few are likely to catch, which showed that this would happen.
Another theory is that one question answers the other. The Yerushalmi in question was Meduyak that for the benefit of what would please the father you may dig him up. The Gra was surprised that the Rebbe Reb Zushe cameto the same conclusion to allow them to open the grave.
When he said that he got it from the same place that refers to the idea that it is a simple Svara. It can also be a tit for tat answer for a remark like that. It can also be that he meant what he said.