This finds are not really a surprise. The XIII century was the apex of the Western Middle Ages as proven by rapid urbanization and the power of free cities, the apparition of universities, the building of large cathedrals and castles, large scale and long distance trade, parliaments and charters, etc.
The great thing about this type of studies is that they don't rely on narratives, but on statistical data bringing hard evidence into debate. Unfortunately this methods are much harder to apply for areas and periods that don't have enough data, including Roman history.
Of course, data must be analyzed with care and correlated with the narrative. For example the great increase of GDP per capita they mention in the mid XIV century is provoked by the Black Death killing a third of population and making the survivors richer, but is hard to see that devastating pandemic as a positive development.