It seems the article isn't just saying it's heart muscle that's being lost but regular muscle in general. Even more so than in a low calorie diet.
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>Studies suggest muscle loss with these medications (as indicated by decreases in fat-free mass [FFM]) ranges from 25% to 39% of the total weight lost over 36–72 weeks. This substantial muscle loss can be largely attributed to the magnitude of weight loss, rather than by an independent effect of GLP-1 receptor agonists, although this hypothesis must be tested. By comparison, non-pharmacological caloric restriction studies with smaller magnitudes of weight loss result in 10–30% FFM losses.
Comparing weight loss of different magnitudes is kind of comparing apples to oranges. Of course, it's not really possible to get persistent, large magnitude weight loss any other way than by using these drugs, so I understand why the comparison was made.