DHS alone would plausibly explain that number. Personally I think it is rather low, maybe they missed something?

Professionals were 59.8 percent of the total workforce in 2020, with 88.4 million people working across a wide variety of occupations.

Agencies listed above currently employ ~250K (millions of former workers though). But lets be gracious and just count currents.

250K/88.4M = .0028

~40K FB x .0028 = 112

> given the number of employees in aggregate those agencies employ and turnover in a given year

The US Federal government is the single largest employer in the US.