You have resigned from the service voluntarily without invoking the grievances against your reporting manager. It appears that the Senior Management was not even brought into confidence before submitting your resignation. You just completed the process of parting away from the company by merely submitting feed back along with your resignation. You have not mentioned the response of HR on the letter of your submission and feedback. It is difficult to comment further in the absence of the said information.
In no way this can be termed as wrongful termination as you did not raise your grievances against the reporting manager at any forum while you were in service. You may write to your ex-employer regarding the vindicated attitude undertaken against you by the company while responding to reference check.