Billy Dee Williams played Harvey Dent in the first Tim Burton Batman film. These are fictional characters. They have no race. It is actually impossible to cast them as the wrong race*.
MLK was a real person whose race is inherently bound up in his contributions to society. Having him be played by a white person would not only be bizarre but would be insulting. A story about a leader of a people rising up for themselves becomes about a pivotal white guy saving brown folk. The entire nature of the story requires MLK to be black. He is not a fictional character.
I have not seen Supergirl, but I'm not particularly invested in the Jimmy Olsen characterization. This is a random and minor side character. He historically exists only to get into weird predicaments that superman has to save him from or otherwise be the lens into some weird superman behavior.
Jimmy Olsen is a relic of a bygone pulpy era of comics that essentially no longer exists. This is not the characterization cross that anyone should want to die on. I have an entire bookshelf (floor to ceiling) full of nothing but comic book trades. I'm easily past the 90th percentile on this board for who has read the most of them, and this is a thing that just doesn't matter.
* I recognize there are some characters in the course of comic book history whose stories are defined by racial identity. Milestone comics characters probably can't be white. Jaime Reyes probably can't be anything but Hispanic. Jimmy Olsen is not one of those characters.