So why did WKA need the 14th? He wasn’t a slave, was he?
Several points regarding this. Wong Kim Ark wasn't a slave, he was the child of Chinese immigrants who were bared by treaty between China and the United States from becoming citizens. Wong Kim Ark could not claim Jus Sanguinus for citizenship, and so they argued that the 14th amendment was operative.
A lot of people argue that Wong Kim Ark was wrongly decided, and that Wong should not have been declared a citizen at all, that there were other means of remedying his situation, and that the Supreme court intentionally disregarded Statements by Legislators regarding the 14th amendment which would have clarified that Wong would not qualify under it's provisions. In other words, the court was legislating from the bench.
I will note that the 14th amendment did NOT give citizenship to Indians until 1924 through an act of congress.