The prophecy given to Rebecca in Genesis 25:23 wasn't concerning Esau and Jacob in their lifetime, Esau didn't serve Jacob.

This prophecy became true regarding the nations that came from them.

Today, the Arabs came from both Ishmael and Esau because the descendants of Esau intermarried with the descendants of Ishmael.
Genesis 28:9
" Then went Esau unto Ishmael, and took unto the wives which he had Mahalath the daughter of Ishmael Abraham's son, the sister of Nebajoth, to be his wife."