The immediately visible characteristics of the frame seems to confirm that it could be an 85 and later model Peugeot. As does the description of the blue paint, as I have seen a few Perthus bikes in the Bleu and white fade Peugeot colors and graphics.. Now, why your frame builder doubts that it has 753 tubing is something he needs to explain better, I think the 753 tubing requires special handling/welding per Reynolds because of the heat treatment on the tubes, but I'm wondering if that really precludes them from putting clearance wheel and chainring "flutes" on the chainstays. Only characteristic I know about the 753 is that you cannot cold set the frame as the heat treatment will just make it spring back to it's original dimensions when you release it, plus I think the tubing air hardens after brazing/welding.

Chombi