Recently viewed movies.

Perhaps there was a scene after the credits. The captain took him there, and at the end, the captain and the Black Panther are standing side by side.

Ah, that explains a lot. Sometimes, I don't have the patience to watch everything.
In any case, I already knew that Barnes would be in Wakanda in the following films, but I expected him to participate in some way, rather than just watching the tribes. But alas.