A few thoughts and takeaways.
1.) This case only applies to employees that the NLRA covers. In other words, it does not apply to anyone classified as a "supervisor" under Section 2(11) the Act. For these employees, keep your severance agreements as-is; this case does not impact them at all.
2.) McLaren Macomb is prospective only. It will have no impact on severance agreements previously signed.
3.) This decision makes very little sense. By the very nature of a severance agreement, you are giving a former employee something to which they are not entitled (additional money) in exchange for a signed agreement that contains certain covenants. The ex-employee is free to take the severance payment in exchange for the signed agreement (including the confidentiality and non-disparagement clauses) or not take the severance payment (and say whatever they want to whom). As these non-disparagement and confidentiality clauses are a condition of the receipt of a severance payment to which a non-employee is not otherwise entitled, I fail to see how the NLRA is implicated at all.