I looked through the whole paper and wasn't really surprised by the findings. Even the dual and triple agonists still showed big drops in fat-free mass. If someone stays inactive after starting these peptides it makes sense that lean tissue disappears. The authors also flag several unknowns that still need study.
Various research papers have praised GLP-1 medications for helping with many conditions beyond diabetes and shedding pounds. This one stands out as negative though. It finds that even though weight comes off with these drugs, aerobic fitness doesn't get any better.