It is probably true that they can axe people in their probationary period without much concern, I find it very hard to believe that the fed would be willing to breach a contract so brazenly within a two month span (Trump admin). I seriously SERIOUSLY doubt that the fed can just axe thousands of BUE's at-will with absolutely no recourse. That's just not going to happen. If the RIF was really gaining traction you would see hiring freezes, early buyouts, etc. None of it makes any business or economic sense.
The FAA is asking for a little over 17 billion for FY 2021, that's a drop in the bucket, that would be 1.7% of a 1 trillion dollar stimulus package.
https://www.transportation.gov/sites/dot.gov/files/2020-02/FY 2021 FAA President's Budget.pdf
Also there are roughly 14,000 ATC's in the USA, if they all made $150,000/yr that would come out to 2.1 billion dollars (and it will be much less than that). Not to mention the COVID pandemic isn't permanent and we already had a nationwide shortage of ATC's going into this year and we've only lost more through retirements/medicals etc.
So why would the fed start axing people who they invest thousands and thousands of dollars to train, to save a few bucks? The answer is they wouldn't.