I think you're saying "how do we know this isn't the normal amount of Phd people who leave every year/administration

While we don't have PhD numbers the Trump administration fired a large amount of people so no matter some portion of those had Phds therefore it must be higher than the previous administration

From the article:

"Science’s analysis found that reductions in force, or RIFs, accounted for relatively few departures in 2025. Only at the Centers for Disease Control and Prevention, where 16% of the 519 STEM Ph.D.s who left last year got pink RIF slips, did the percentage exceed 6%, and some agencies reported no STEM Ph.D. RIFs in 2025."