I think the "knights" referenced by the questioner were KotCs, not Knights of the Fae Courts. Murphy and Susan are both examples of KotC that hold a sword for a short time; both during the Chichen Itza assault (yes, I'm saying Murphy has already been a KotC, a 'one knight stand' so to speak ). Rapid changeover in KotCs doesn't, to me, provide Harry any 'out' from his position.
The question references Winter directly so idk where you get the idea that its
not talking about the Fae Knights. Jim may have answered regarding the KoTC but that seems more like a misunderstanding on the questioner's part when he references what the author had said before and Jim just auto corrected that notion in his answer, or he misunderstood the question.
In that WoJ Jim is specifically talking about both Knights of the Coin and the Fae Court Knights, though the court knights are typically a "lifetime" endeavor. i.e. they generally don't live long.
And he's working from my simple idea that if Mac got an "out" as a starborn, the same option would possibly be available to Harry. I think the choice hasn't presented itself yet, but I do think it's coming, of course, no choice is without consequence, and it will most definitely prey on Harry's personal nobility and sense of honor.
You referenced the WOJ about Starborn to support your idea that Mac was also one, I provided it for you simply to add to your post if you wanted...my issue with that idea is that you attribute the evidence by asking who exactly would be able to be 'out' and then give that ability to a Starborn. And since we know Harry is a Starborn I think that is not a very strong argument because it pre-supposed that Harry will have the same ability to be 'out' because he is a Starborn (intentionally or not that's how the idea is presented). And I think that from a Doylist view that would be far too easy an out for Harry and Jim would never allow it, and from a Watsonian view it is unsupported in the story because, by your model, a Starborn who is 'out' was very much brought back in, being attacked, kidnapped and shot. That at the end of the day him deciding to 'not' be a Starborn for all intents and purposes didn't do anything to keep him uninvolved. That was my only quibble.