Inflation vs GDP growth as factors driving down the US debt-to-GDP ration post-WW2
Look at the implicit price deflator for GDP. Using the annual series, 1946=11.05, 1956=15.32, so 38.6% increase. (Source: FRED database)

I don’t have time to look at the paper, but that’s possibly what was meant. Not an entirely convincing methodology, but you would have to do something with the deflator as well as nominal GDP to allocate changes.