Is that true? I've never seen/heard of a quality bike with a reinforcement insert in the seat tube. I think that the variation in required seatpost diameter is a result of

a) the thickness of the tubing used -- some tubing manufacturers used single butted tubing on the seat tube (meaning that the top of the tube as constructed would be thinner rather than thicker gauge. Or they recommended single butted for smaller frames, and made double-butted available for larger frames (Reynolds). And some mfgrs (Columbus, Tange) seemed to use DB tubing for seat tubes.

b) French bikes have incrementally smaller outside diameter tubing -- even metric sizing rather than Imperial. So while an English or Italian frame would have a 28.6 mm OD seat tube, a French frame would have 28 mm OD (and take a correspondingly smaller diameter seatpost.

So, Raleigh 531 frames might take a seatpost 26.4 or 26.6 mm if built in France, while an English/Italian bike built with tubing having similar wall thickness (but Imperial sized) would take a 27.0 or 27.2 mm post.