Well, that's where we get into America's particular preoccupations and perceptions of what "race" means. Because of the way slavery and its legacy shaped our history, Americans have long had a tendency to perceive race in dualistic terms -- "black and white" in more ways than one. "Interracial" tended to be seen as synonymous with "between black and white," with other racial categories glossed over. That's because, in the '60s, other interethnic romantic pairings weren't as controversial or shocking as black-white unions. You could have Lucille Ball and Desi Arnaz as a couple in the '50s without much controversy, not to mention Shatner kissing the French-Vietnamese France Nuyen in "Elaan of Troyius" (which was filmed before "Plato's Stepchildren" but aired later). But a white person kissing a black person was a drastically different matter. After all, it had been only a year and a half earlier that the Supreme Court had struck down all anti-miscegenation laws which barred marriage between blacks and whites in many states of the union. And several states kept those laws on their books even though they could no longer legally enforce them.
So no, maybe the usage of the word "interracial" there is not technically accurate, but there's a lot more at issue here than vocabulary. A black woman and a white man kissing on TV in front of millions of viewers was a really, really big deal in 1968, because there was still a lot of blatant, hateful racism in the country. So yes, even with the vocabulary quibbles, it was a major landmark.
I agree with many of your comments here (your entire post). You make great points. However, I partially disagree, or at least I think an asterix is needed where you brought up interracial relationships involving a non-black
Particularly when addressing the French-Vietnamese actress. What would have been considered more appalling, even today for that matter, would be the idea of an Asian male actor kissing a White female actress, than a white male kissing a black female.
And not only would, say, Sulu kissing an attractive white actress be out of the question (not just to George Takai); but Sulu kissing France Nguyen would have been out of the question. American media seems to have this strange idea that Asian males don't interact with females of any race including their own.
As far as your point about TOS not being revolutionary, but progressive, I think it's a good point, and probably correct.