I think the OP brings up a valuable point. I have just re-read Ctein's book again and confess:

1. I didn't fully understand the section but was astounded by the figure of 15mm, thinking as Michael did that this would show up as plain as your nose
2. I still cannot figure out why this kind of problem didn't spread worldwide alarm at the time of his publication and protests to paper manufacturers. You'd think that it would have entered into the annals of darkroom disasters and still be talked about

3. Is it possible that whatever was causing the problem that Ctein saw could somehow have been peculiar to his set-up, unlikely as this seems? Might paper manufacturers have since done something to rectify the issue?

Ath had no issue and neither or so it would seem, did the majority of users so what was happening in Ctein's tests?

Unfortunately he does not go on to say why it may be that most users will never experience the issue and yet the very fact he devotes a section of his book to it indicates that he thought it a serious enough issue to put it before his readers.

Actually there are a number of "alarms" raised by Ctein in the book which does not seem to have engaged the darkroom world such as serious issues with, I think, some makes of RC paper which he cites and which should be a separate thread. Again maybe the issues he and a fellow photographer of high standing experienced were not that widespread and have been subsequently solved.

I haven't got the date of the book but it would appear that he was speaking of issue in the mid 1990s