'10 years of boom; 10 years of gloom.' how valid is this assessment of the period between the two world wars?

Simply because the 20s were known as the "Roaring Twenties" due to the economic boom in the US which ended with the Wall street crash in 1929 and then 10 years of gloom until the begining of WW2 in 1939 ended the great depression and the USA began getting the unemployed into the factories making weapons (Roosevelts "New Deal") for foreign countries such as the UK and the USSR , Also known as Lend Lease.