That is not what scientists do, as far as I know.
Instead, we look to test hypotheses with data, and we are very careful not go beyond what the data tells us when making scientific claims. Plausibility, after all, is in the eye of beholder. Skepticism as you are using it is selective. The same skepticism could and should be directed to all positions, including yours.
I’ve already said that the evidence is not going to tell us one way or another. I do not think there is a strong evidential case for a bottleneck, or against it. Science is not good with singular events in the distant past. So this should be no surprise.
You are not reading it correctly. (no suprise because you are not reading it)
Varki is arguing that there may have been largely smooth development of the human mind, but at some point there was a very difficult to cross barrier. The indirect evidence he marshals to this point is that there are no other animals with a full theory of mind. None of our cousin species seem to have achieved it, so it must be hard to achieve. If it is so adaptive too, why is that? His resolution is that there is a difficult barrier when it first arises that usually ends it before it can become adaptive. It is a very unlikely event.
That is a reasonable theory. Very difficult to test, but its just as “plausible” as yours. And if this is the case, central to thesis is that there is a sharp difference between before and after in our minds. A sharp difference that can easily account for the absence of interbreeding. He even explains this at length, all from a secular point of view.