It's still a theory. We definitely don't know the underlying mechanism(s) of action, and it's likely there's more hidden complexity there.

But rapid weight gain after weight loss (until you arrive somewhere near your old weight) is at least a well observed experimental effect. About 80% of people who lose weight, through any means, will revert back to their old weight.

Source:

https://www.ncbi.nlm.nih.gov/books/NBK592402/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673773/

There are lots of physiological parameters with set-points, such as body temperature. The problem for weight gain/loss is that instead of one set-point for body weight itself, you have maybe 5-10 set and operating points that are indirectly related to body weight, but not direct measurements of it. They don't all have to be "working right" to keep you healthy, but if too many become disordered at once, you're gonna have a problem.

Set-point theory is pretty much settled medical fact. The mechanism involves leptin, and you can easily see processes that defend bodyweight change in both directions. Though it will more aggressively defend weight loss than weight gain.

In addition prey animals will defend against weight gain more aggressively than non-prey animals. Which makes sense from an evolutionary standpoint. If a lion gets fat he doesn't have nearly as much to worry about than if a gazelle gets fat.