If the employee meant compulsion in that separate statement then the employee should have said so. One generally assumes that people use the words they choose to use. Here, had duress really been used then certainly the employee was compelled to sign it, the statement to the contrary in the contract notwithstanding. The thing is that the statement in the contract that it was signed free of compulsion is not worth a lot to begin with. The former employee could always raise the issue of duress or compulsion (if that term is relevant for executing this kind of contract under state law) in a challenge to the enforcement of the contract. In such a defense, however, the employee would have to produce evidence of the duress or compulsion used to obtain his signature on the contract. The separate statement he sent the employer claiming he signed it under duress is hearsay and not admissible when offered by the employee to help prove he/she signed under duress/compulsion. It is because it is not admissible for that purpose that it is essentially worthless.
The employer is free to send a new contract out and tell the employee to sign it again without any qualifications like the statement about duress or the employer will not pay the severance. There is a risk, though, that the employee having signed the contract makes it enforceable and the failure to pay the severance would put the employer in breach.

