Losing glycogen stored in muscle is not a huge issue IMO, as it should come back fast. Stuff that's easy to gain is usually easy to lose and vice versa.
The point is that there is a big difference between depleting the store of glycogen, which can reliably be refilled in about 2 hours and the body's disassembling half the muscle mass, which takes many months to build back up if you even can build it back up to the original mass (unlikely if you are old).
No one is disputing that you can restore glycogen or water weight quickly.
But the issue is all of the studies I have looked at look at total FFM which does include the loss there. If you are on these GLP-1s there is water weight you are going to lose and keep off while on them due to the anti-inflammation effects, etc., and that water weight is going to be part of their calculations of FFM that has been lost.
