Bruce,
I've not seen it documented in other manuals or resource material. However one must bear in mind that those sources all make the assumption that regular maintenance has been performed, including brake fluid flushes. Thus the problem of water-saturated fluid would not be an issue and so corrosion would not occur in the bore of the cylinder. HOWEVER, since so many of our Volvos are outlasting the majority of vehicles on the road, and since in reality brake fluid flushes aren't ever done on most cars until there's a problem (rusted line for example, here in NS a car with steel lines gets a full flush every 5-8 yrs since that's the life of the lines) there's a very good chance that the fluid does have a lot of moisture and the end of the bore where the seals never travel will have corrosion. I have had master cylinders fail on 3 cars where I've bled the fluid and pushed the pedal all the way to the floor. Chances are it was marginal and would have failed soon anyhow, but the rust did in the seals. I took the first 2 apart to find out what happened and indeed the seal lips were torn and there was rust in the bore.

So I'd say it's not a myth but a probability unless you know the fluid has been changed on schedule and not neglected. Why our cars? As I mentioned the cupro-nickel lines mean that in many cases these cars never get a full fluid flush because only the flex hoses get changed and the m/c topped up with a little fresh fluid. And the master cylinders do last pretty well indefinitely until they're disturbed, so many of those seals are 20+ years old and don't have the flexibility to tolerate grit.

I took one apart one time, before installing it on another car and attempting to bleed it, and I honed the rust out of the bore then reassembled it with the original seals (foolish yes but I was a starving student at the time). Worked like a charm for the 2 years I had the car.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina