My sense of Cushman's ratings claims is that they were better than they've been purported to have been, not that the show was a hit.
I think this is the real point. For decades the story has been repeated that TOS was a ratings failure and justification for NBC wanting to cancel the show. But we've already seen NBC quite liked the show and it was Desilu suits who didn't care for it. And now it just might be, that while not a huge hit, TOS' ratings were distinctly better than what everyone has believed all these decades. It certainly wasn't anywhere near a failure.
There's an analogy for that today: opening weekend box office sales for feature films. The expectation is for huge opening weekend sales and anything else is simply icing on the cake. Whereas at one time films could run from several weeks to months on end at the theatre. Now you're lucky if a film goes a month a the theatre and then the expectation is to make money on the video sales.