If you have informed the defendant that you never worked for, and never applied to, this employer, and the defendant refuses to withdraw the subpoena (or explain why it is relevant), then you should include it in your motion to quash. It will show the judge just how unreasonable the defendant is regarding the subpoenas. Of course, you should probably seek the assistance of an attorney in drafting your motion to quash and to make sure you have followed all the necessary steps in filing and before filing your motion (e.g., the "good faith meet and confer" requirement).
Good luck to you.
You should consider whether there are grounds for a motion to quash. Grounds for a motion to quash include defects in form or content of the subpoena (e.g., inadequate description of requested documents in “records only” or “records and testimony” subpoenas); defects in service of the subpoena, including tender of fees; records sought not within permissible scope of discovery—i.e., privileged, privacy or attorney work product; or not “relevant to the subject matter”; unjustly burdensome or oppressive demands; with regard to requested electronically stored information, objection to the specified form for producing the ESI (CCP § 1985.8(c), or objection to production on the ground that the ESI is from a source that is not reasonably accessible because of undue burden and expense (CCP § 1985.8(e)); “Consumer's” right of privacy in “personal records” (CCP § 1985.3(e); or employee's right of privacy in “employment records” (CCP § 1985.6). See Rutter Guide, Civ. Proc. Bef. Trial, 8:598. It is difficult to see how your scenario establishes any grounds for a motion to quash since there presumably are no records to obtain, thus no possible invasion of privacy, nor any undue burden or expense.
There may be more to your story, so I don't profess to provide an answer to your question, only to suggest that you consider how you would argue to a court why the subpoena that you believe will return no records from your non-employer should not be answered by the non-employer.
Answers to questions are for general purposes only and do not establish an attorney-client relationship between the inquiring individual and this responding attorney.
They are probably seeking to confirm your representation that you never worked for or applied for work with this employer. They are entitled to do that, and it is not an abuse of their subpoena power to request make a request for this purpose.
If you truly had no involvement with this employer, then why should the subpoena even concern you? It's not worth putting the time and effort into filing a motion to quash if there is nothing you need to prevent the other side from discovering.
This answer is a general interpretation of the law and is not fact specific to your case. Likewise it does not create an attorney-client relationship. You should seek an attorney for a review of your specific facts and documents.