It might not be that far fetched.
Consider that there are variances in all the elements involved - shutter speed, aperture opening, film speed rating, film development. I suppose these variances can work out to cancel each other, but can also add up. 3 stops off is quite bad, but I won't be very surprised if 1-1.5 stop unsuspected difference is not possible to happen this way even on seemingly ok equipment.
Sure, a stop and a half is credible -- if, as you say, you incorporate
all variables and they
all go in the same direction -- but it sounded as though the author was talking
only about the camera. And even if you are considering all variables (and add sloppy metering to boot), 1/125 to 1/40 (1.5 stop shutter speed compensation) is not quite the same as 1/125 to 1/15.
Many years ago, the head of a major importer in the USA told me about selling cameras via "the fear factor", summed up as "Well, yes, your camera can do that, but can it do this?" "This" was of course a feature of the camera you were trying to sell.
An awful lot of the Zone System strikes me as very similar: looking for a precision that isn't there, and wouldn't matter even if it were, but relying on people's innate fear that everything looks too easy if they don't use the Zone System.
Once a negative is adequately exposed, it is adequately exposed, and the penalties for doubling the exposure are negligible (slightly bigger grain, slightly less sharpness, both completely irrelevant with large format and substantially irrelevant with rollfilm). The risk of blown highlights is negligible: look at any manufacturer's d/log E curves.
A LOT of Zonies are actually relying on their chosen films' (and formats') tolerance for overexposure, making up for sloppy metering and underdevelopment (the latter seemingly an article of faith) with more or less gross overexposure. Critical metering is meaningless unless you read the shadows with a spot meter, because that's how film speeds are determined, but there are Zonies who use incident light meters...
Yes, the Zone System (or basic sensitometry) can teach you a lot, if you understand what you're doing; but many don't, and treat it as a religious ritual.
Cheers,
R.