It is, of course, always harder to make a diagnosis from pictures than hands-on. There does appear to be funneling at each end of the burst & a bulging of the bottom rib, though doesn't appear to be in the top rib.
As to "Pistol" powders, shotgun powders & pistol powder have a lot of overlap. There are shotshell loads using Bullseye & pistol loads using Blue Dot & virtually all powders with burning rates in between.
I assume the mentioned substitution of pistol powder was meant as an overload of fast pistol powder in place of a heavier load of slower burn powder. An overload of fast powder should result in a chamber burst, not down ahead of the forend. Some other factor may well have been involved in that case which was overlooked once the powder mistake was discovered, hard to say without more info.
This barrel in the pictures does not look to be overly thin. It may well, however, have been struck thinner on the outer portion that the inner, which could account for the top rib not being bulged. I do not see it as being the result of "Tired Steel".
