Ge 28:6 Esau saw that Isaac had blessed Jacob and sent him away to Padan Aram to take himself a wife from there, and that as he blessed him he gave him a charge, saying, "You shall not take a wife from the daughters of Canaan,"

7 and that Jacob had obeyed his father and his mother and had gone to Padan Aram.

8 Also Esau saw that the daughters of Canaan did not please his father Isaac.

9 So Esau went to Ishmael and took Mahalath the daughter of Ishmael, Abraham's son, the sister of Nebajoth, to be his wife in addition to the wives he had.

Esau tried to win back Isaac's blessing by marrying one of Abraham's line. In doing so, he only confirmed his unchosen status by marrying into the line of Ishmael. It was Isaac who received the blessing of Abraham, not Ishmael.

Moses is showing his original Israelite readers the established fact of their chosen status, being descendants of Jacob, and not that of Esau.