The study found that heart muscle decreased in both lean and obese mice. So any observed muscle loss might not be just from losing body mass and not having to work as hard.
But if you're already lean and then go on a calorie deficit (as a result of decreased appetite from taking the drug), then muscle mass will be lost through metabolism of muscle and other tissue.
Then the study states further that the proportion of muscle loss is higher than expected from calorie restriction alone.
My gut feeling here is that where there's smoke there's fire, and I predict dramatic class action 40 years in the making, either like tobacco, or like baby powder, depending on the actual long term health outcomes.
And, this is great research! We need more like this ASAP!