It's not so much a question of it being binding on the insurer, since the insurer is not a party to the severance agreement (a party cannot be bound to an agreement to which it is not part) as the fact the agreement is binding on the employer--they need to make sure you get the coverae, if that's what they agreed to, or else potentially be liable for medical costs for that month. The above, said, it would be prudent, before getting the surgery, to check with your former employer that they paid for the month, and to check with your insurer that they actually have you as covered for that month.