score:6
This publication from 1920 seems to have taken a look at the millionaire claim:
There was definitely a boost in the economy, as war requires and consumes tremendous material production, which can be seen looking at the USGDP numbers for this period. Note the stagnant years before the war, but production increased steadily after:
1910 33.4
1911 34.3
1912 37.4
1913 39.1
1914 36.5
1915 38.7
1916 49.6
1917 59.7
1918 75.8
1919 78.3
1920 88.4
...and 1916 did see the worlds first billionaire in John D Rockefeller, but the 20,000 millionaire claim may be a manipulation or misinterpretation of statistics.