Aaq, that’s not how a kasha/terutz works in any beis medrash. We take for granted that the avos knew such things; avrohom kept eruv tavshilin, though it was a gezerah from chazal.
According to you, yaakov knew that there is a zchus of living on eretz yisroel, but didn’t know that it was promised to him and not eisav with the brochos? It was said to him by yitzchak clearly.
But that’s besides the point; we don’t need the drasha of ki B’yitzchok to know that the brochos, which yaakov had and which eisav lost, guaranteed him eretz yisroel, to the exclusion of his brother.
What does that have to do with misas ha’emes? That he can see a zchus which is….not a zchus, because it has nothing to do with the mitzvah of yishuv EY?