The argument presented by Sam Jackson, while poorly worded, is valid in a lot of ways.
Similar to the claims of Transgendered people who ask, “why didn’t you cast an actual trans person to play the role?” in the case of the show Transparent. Or when gay people wonder why a straight person is cast in a gay role?
These people have actually lived the experiences of the character and can/should in theory be able to bring all of that experience into the role.
Issues of the divide between british and american black actors are valid though and I do agree with Boyega’s tweet. But that agreement doesn’t make what Samuel L. said any less true, even if he did word it poorly.