I guess that this journal does not even send out rejection letters. It if did, you could consider using this Rejection of a Job Rejection letter as a template for a reply:
Rejection of rejection letter
Dear Sir or Madame,
Thank you for your letter of February 9th. After careful consideration, I regret to inform you that I am unable to accept your refusal to offer me
a job in your department.
This year I have been particularly fortunate in receiving an unusually large number of rejection letters. With such a varied and promising field
of candidates, it is impossible for me to accept all refusals.
Despite your department's outstanding qualifications and previous experience in rejecting applicants, I find that your rejection does not meet my needs at this time. Therefore, I will assume the position in your department this March. I look forward to seeing you then.
Rejection of rejection letter
Dear Sir or Madame,
Thank you for your letter of February 9th. After careful consideration, I regret to inform you that I am unable to accept your refusal to offer me a job in your department.
This year I have been particularly fortunate in receiving an unusually large number of rejection letters. With such a varied and promising field of candidates, it is impossible for me to accept all refusals.
Despite your department's outstanding qualifications and previous experience in rejecting applicants, I find that your rejection does not meet my needs at this time. Therefore, I will assume the position in your department this March. I look forward to seeing you then.
Best of luck in rejecting future applicants.
Sincerely, <you>, Esq.