We are about ~300 years closer to Diocletian than Diocletian was to Ea-nasir. Too bad he didn't have access to Adam Smith, who could have told him that price fixing wouldn't work and might have pointed to Roman currency debasement as one of the major causes of inflation. Of course greed is always a factor I suppose.
Lots of people say lots of things. I don't see why Adam Smith is special. Price fixing can work within reason.
Did you know that the communism-themed Fusion Festival fixes the price of a meal, across all 100+ vendors, currently at 7€? This is presumably negotiated between the festival organisers and the food vendors as a reasonable price. (Some of them make a point of advertising 6.50€ but that doesn't actually make a difference. If you like the food you would've bought it for 7€ and if you don't like the food you aren't going to change your mind over 0.50€. They're just throwing away money.)(Some stands that sell more "snacky" items like crepes or smoothies use lower prices.)
Because the price is the same everywhere you don't have to comparison-shop and can just pick the food you like.
Fair! Chronologically, at least, Diocletian's time is closer to us, though not technologically or population-wise. And by Diocletian's time, inflation was already out of control. If anything, the political stability that came with Diocletian's reforms actually helped bolster the economy, though it wouldn't last, as soon the center moved to Byzantium, leaving Rome (and Italy) to whither and die over the next century.