Diocletian's Edict on Prices, which threatened citizens with death for coming to a voluntary agreement on goods and services, was certainly immoral and unjust, and it must have had a negative economic effect as well. When prices are set artificially low, producers stop producing, leading to shortages. When prices are set artificially high, consumers economize by lowering their purchases of other goods. This is the basic law of supply-and-demand, which should be no more controversial than the law of gravity or evolution by natural selection.
Moreover, according to Gibson's analysis, the state had lavished the wealth of the state on economically unproductive churches and monestaries, thereby transferring support from the military to idle mouths that contributed nothing more to Rome than removing themselves from the gene pool. What could be controversial about the economic effects of such a policy? By removing money from the capital markets, the state must have made money less available for loan for commercial enterprises, thereby increasing the interest rate and reducing the likelihood of taking risks on overseas trade, new business ventures, and the like. Again, this is merely basic economics, which should be no more controversial than the law of gravity or evolution by natural selection.
What SHOULD be controversial, however, is whether the economic policy of the late empire was sufficient to cause the historical decline that was observed. In fact, the material evidence from archaeology depicts a high degree of luxury all over the empire UNTIL the barbarians invaded. Thus, rather than economic decline being sufficient to explain the fall of Rome, it seems instead to have contributed to the inability of the Empire to finance a sufficient defense against the barbarian hordes, which toppled Roman provinces in Europe, North Africa, the Aegean and Levant, and finally Asia over widely-spaced intervals of time. The timing of all this can't be explained simply by reference to economic factors alone.