POA for incompetent executor of third person's estate

I have power of attorney for my father, who is no longer competent. He was executor of my mother's estate. Dad was behind in filing income tax returns; his tax advisers are ready to file his return for tax year 2023, during all of which Mom was still alive. They are asking for e-signatures on form 8879 authorizing them to file a joint return.

The tax advisers say, "Since you are acting as attorney-in-fact, you may sign the e-file authorization forms on their behalf. These forms are for our records and authorize us to electronically submit the completed tax return."

Are they right, even though I never had any legal authority to sign for Mom? I guess I'm asking in general, does a power of attorney document that authorizes the agent to sign tax returns for the principal also have authority to sign them for a third person for whom the principal was authorized to sign?