Is criticizing an expert's qualifications an ad hominem? Particularly when they're using their expertise to argue for an idea as here. Seems like a double stan

Is criticizing an expert's qualifications an ad hominem? Particularly when they're using their expertise to argue for an idea as here. Seems like a double standard. They can use their expertise/reputation to endorse an idea but using that same expertise/reputation to qualify that endorsement is an "ad hominem." To me discussing their expertise is core to their position, since their whole position is: "I'm an expert, I sign a letter based on my expertise for a specific course of action." If they aren't an expert it cuts right to the core of their position.