Dewey states many strong opinions, some better than others. But all held firmly and he has his reasons. While they might not work well for him they still seem to be held in high regard by those in the trade in UK. Don’t forget wall thickness is not part of in proof or out of proof. It is a dealers/buyer attempt to figure how much life is left in a set of barrels and how much of a dent risk they present.
I just finished shooting 2,000 20 gauge shells in a Pape hammer gun with “ barrels below recommendation”, but still in proof. They have one area of .016 that is 27” from the breech face on a set of 30” barrels. A real dent risk but not so much a safety issue due to location. I even found a decent lower pressure load which breaks Sporting Clays well so I might load up another 2,000 shells to run it for a bit longer.