I don't claim that the notion is canonical. It's obviously easier for producers to treat it all ("Kelvinverse" aside) as one single timeline from beginning to end. As I said way back on page one of this thread, though, I don't really care about "word of god" in my filmed entertainment (and even less in Trek, where it could change from one production to the next); once something's out there, it's open to interpretation.
It's a matter of headcanon. Trek is chock-full of time-travel incidents that, to my mind, don't logically leave the timeline in the same shape it started in. (I already mentioned STIV and "Yesterday's Enterprise" earlier, but there are plenty of others.) FC is one such that just happens to be at the right time, and to the right time, to explain away the existence of a subsequent series that retconned a whole lot of TOS continuity (albeit not to the same extent as DSC, FWIW), yet had somehow never been mentioned before. It seems to me more logical and more elegant to regard it that way than otherwise.
It's a matter of headcanon. Trek is chock-full of time-travel incidents that, to my mind, don't logically leave the timeline in the same shape it started in. (I already mentioned STIV and "Yesterday's Enterprise" earlier, but there are plenty of others.) FC is one such that just happens to be at the right time, and to the right time, to explain away the existence of a subsequent series that retconned a whole lot of TOS continuity (albeit not to the same extent as DSC, FWIW), yet had somehow never been mentioned before. It seems to me more logical and more elegant to regard it that way than otherwise.