This is a pretty cool achievement for them. The public sector and government work as a whole has always been pretty well unionized (by US standards anyways), but even that space has taken a bit of a downturn in recent years according to the BLS. Attrition is also a huge and constant issue for unions like this, as many members turn over each year.
In this respect AFGE is pretty impressive: they've posted strong growth and retention in membership in the past two decades. They counted 232,000 members in 2008, 288,000 in 2014, and 294,000 during the middle of 2015. The Trump years and pandemic were apparently pretty bad for them and in 2022 they counted "only" 282,436 members to end the year—but it seems throughout 2023 their growth has once again skyrocketed to reach the 300,000 members they're claiming this month.
Note that their actual membership is also only about half of the total number of employees they represent the bargaining rights of—they last claimed to bargain for approximately 750,000 members across 900 local unions, although I don't have a date for that number.