I can't speak for the other poster, but the way I see is is that "forced inclusion" is where the script directs viewer attention to it in a protracted, unnatural manner that is not pertinent to the plot. For instance, the script may be as blunt as a character saying "Wow, I can't believe you made it this far despite being a [marginalized out-group]," or it could be a little more subtle by offering a stereotyped representation of [marginalized out-group] without any kind of deeper exploration. i.e. Tokenism

Star Trek, for the most part, dove into social subjects deeper, more meaningful way than other media at the time. Like other users have pointed out, TOS confronted racism and gender roles head on by placing a black female character on the bridge. By never drawing attention to those traits, the show issued such a strong rebuke against racism and male chauvinism that no more needed to be said. In my view, that is inclusion that is not forced upon the viewer; it is implied, but unless the viewer is explicitly looking for it, they'd never notice.