It's just one of those things which I like to call 'ascended fanon', ie it's a fan theory which has taken on a life of its own and been
effectively accepted as a viable and official part of the universe despite there being no direct evidence for it in any screen incarnation, while there being nothing to contradict it either.
Even if it had been thrown around in fan circles before-hand, I think it was the Okuda's
Star Trek Chronology that fed the flames on the blond lab technician being Carol Marcus (as it so often did with this kind of fanon..... note that so many entries in that book are labelled as being "Conjecture", but because of the weight of the Okuda's involvement in Trek at the time, many of us accepted these 'fan theories' in the absence of anything more official being established on-screen).
There are a lot of things regarding TOS that are in fact 'fanon' in this regard. Some of them have subsequently become facts established on-screen, while still others never have, but are, for lack of anything else, "accepted" by the fandom as a kind of 'second tier' continuity anyway.
You are so right! It's just something that happens and some choose to believe it and go with it as it is never addressed or referenced again. It would be a conjecture. It seems that whatever is the most accepted solution tends to be what is followed. The only way for it to be canon is if it is addressed onscreen
To address the 'Okuda thing' I mentioned for a moment..... there are some things, for example, that the Okudas' both came up with as being "conjectural" backstory for the TNG characters, which they then used as the basis for the computer biographies we see on-screen in 'Conundrum'. So on that level, the fanon becomes the canon, because those of us with good eye-sight and freeze-frame would, in theory, see those particulars being stated as 'fact' in an on-screen context.
Of course, it works the other way too. Sometimes we accept the fanon as valid (and authors may even write books based on it), but it then gets contradicted by on-screen material, in which case the fanon obviously no longer has any weight at all -- this happened to the Okuda's with a whole boat-load of "conjecture" over Zephram Cochrane and the first warp flight, most of which was published in the first edition of the
Chronology and
Encyclopedia, but got ignored when they made 'First Contact' so it was struck from the subsequent versions of those books. And then you've got novelisations, and delete scenes from filmed material..... they've got
some degree of validity, novelisations are usually based on earlier drafts of a script and contain material that might but for Grace Of God have appeared in actual canon, and yet ultimately it is still secondary material, because for whatever reason it got *cut* from the final product.
I think it's healthy to be open to fanon, like the blond lab technician
maybe being Carol Marcus.

But I also think it's the sort of thing that shouldn't be
assumed to be true.
It gets harder still when one version of on-screen evidence contradicts another. For example, are we to accept the version of Cadet Picard with hair that we actually saw in flashback in "Tapestry", one which on the face of it was at least
consistent with everything we assumed about Picard to that date? Or do we erroneously accept the bald Picard photo seen later in the movie "Nemesis"? That one is a dilemma for the ages.

Is the most recent material in some way 'more relevant' than the older material, even though both have got
some claim towards being true?

Or do we accept the older material over the newer, simply because the older material got there 'first' so it has seniority? Even I don't have an easy answer to
that one.....
