Declining an unreasonable request from a superior
Your reluctance is understandable. 

Write to the editor and let him know that while his/her reputation would have standing with these authors, your own would not. It does no one any good if your letter to the authors is sent but ignored. 

Offer to write a letter to them, but for the signature of the editor to be sent officially. The authors are much more likely to wait given a request from the editor. Include anything you think needs to be said, but from the editor's standpoint, not your own. 

An alternative, perhaps nearly as good, is to start out your own letter as "I have been instructed by Dr X, editor, to write to you with the following request and instructions...."