But the real world usage of disgruntled is as a marker for "This person is biased against the company, so any fair-minded person should discount what they have to say."
Ad hominem doesn't mean "calling them a name or label that isn't true"--it means "implying that their argument is less valid because of their state of mind or perspective".
All of the above, and I really don't see the ad hominem in there. It's just a description of a state of mind with respect to another entity.