The data had already been declassified up to 2020, so they only needed to estimate changes over the past 3-4 years:

> Between 2010 and 2018, the US government publicly disclosed the size of the nuclear weapons stockpile; however, in 2019 and 2020, the Trump administration rejected requests from the Federation of American Scientists to declassify the latest stockpile numbers (Aftergood 2019; Kristensen 2019a, 2020b). In 2021, the Biden administration restored the United States’ previous transparency levels by declassifying both numbers for the entire history of the US nuclear arsenal until September 2020—including the missing years of the Trump administration.

Source: https://www.tandfonline.com/doi/full/10.1080/00963402.2024.2...