They've overplayed their hand and are now forced to buy people out to deliver on their objective. If they could just fire them, they would, but they can't.
After re-reading the letter this was a very slick move.... they never promised anything more than (very obtusely) the $25k. The article's wording has been repeatedly edited and the Editors Note has changed a few times as well: "Editor's note: This story has been updated with background on the administration's memo.", "Editor's note: This story has been updated with confirmation from the Office of Personnel Management."
If I’m reading this correctly, it isn’t a severance package, exactly. The only “compensation” an employee is getting in exchange for agreeing to resign in September is the ability to continue working from home. Things are messier if agencies start placing those employees on paid administrative leave etc.