I think the explanation is correct.
This being said - and I can only go by the photos - I don't think the barrel actually looks deliberately thicker at the breech. Rather than there being a deliberate 'bump' at the breech, I think it looks like a long-acting corrosion 'dip' just forward of where the protective plate would have been. In other words, the protective plate kept the barrel from corroding just in that area, and immediately forward of it, there was some corrosive spillover which ate away some of the material. At some point the plate was removed and the barrel likely cleaned up a bit, and now the formerly-protected area stands prouder than the formerly-corroded area. Following the flat lines back from muzzle end to breech, it sure looks to me like there is a dip in that area just forward of the pins. Which btw, I would suspect are screws that attached the plate and then heads filed off into a small countersink.