I don't want to say fitness addiction isn't as legitimate as the ones you listed--but I do think it's less "legitimate" if only because the others are actively detrimental to several aspects of your life and health. Fitness addiction is bad if you consistently get injured, but otherwise it's not nearly as immediately, physically detrimental.
You are right in the other ways. If it leads to you misplacing your priorities and doing it at the expense of other things you're gonna wind up in a bad place. As was said, all things in moderation and there's absolutely no panacea for a problem, but doing something that can help you feel better about your body during a down period can help! There's often a feeling of no longer being in control after you're dumped because your "ideal" life was just changed radically, and doing something that helps you regain control over your life in a very tangible way has loads of benefits.
I won't vouch for it during an actual depressive episode, though. Fuck people that tell you to eat healthier, work out more, or do yoga

while you're mentally fucked. No shit, sherlock.