Companies can't shut you up using severance pay, rules NLRB

The NLRB ruling can be found on the NLRB website at https://www.nlrb.gov/case/07-CA-263041 . (I find it interesting that the NLRB site shows nothing between the ruling on 2023-02-21 and a response to questions back on 2021-12-10. That's a really long window, but I don't know how this system works.)

The ruling itself spends a lot of space discussing how the union's non-involvement with the furlough process was illegal. This makes sense, as the National Labor Relations Board (NLRB) is supposed to deal with union stuff.

I'm not a lawyer, and I'm unconvinced by my cursory reading of the ruling that "companies can't shut you up using severance pay" is true in all cases, or if it is specific to unionized workforces. The Register is not an organization I expect to have legal expertise, though I could be wrong. Does anyone know if The Register actually has legal chops, or if there is a more lawyerly source on 07-CA-263041 ?