The Hebrew text of Psa. 89:27 literally states,
Even I will make him firstborn, higher than the kings of the earth.
One who is “firstborn” has pre-eminence above all others.1 Just as the nation of Israel was Yahveh’s firstborn,2 and Yahveh would make the nation of Israel “high above all nations that he made,”3 the Messiah, who is David,4 just as he is Israel,5 Yavheh would make firstborn, “higher than the kings of the earth,” for one “David” said in Psa. 18,
43 You have delivered me from the strivings of the people; You have made me the head of the nations; A people I have not known shall serve me. 44 As soon as they hear of me, they shall obey me: the strangers shall submit themselves unto me. NKJV, ©1982
David, the son of Jesse, was not the head of the nations, but only the head of the nation of Israel, nor did other nations serve him. On the other hand, the Lord Jesus Christ is higher than the kings of the earth, and all nations serve (or shall serve) him.
2 Gen. 49:4–5 cf. Col. 1:18
2 Exo. 4:22
3 Deu. 26:19
4 Jer. 23:5, 30:9; Eze. 34:24; Hos. 3:5
5 Isa. 49:3; Hos. 11:1 cf. Matt. 2:15