Posting this because the summary going around does not say what the paper says, and the difference matters for how people here are using it.
There is also a quality change that the scan cannot see. Weight loss shifts fiber-type distribution toward Type I and improves mitochondrial density and insulin-stimulated glucose uptake per fiber — so the muscle you keep is metabolically better even though there is less of it. That does not make the loss irrelevant, but it explains why function often holds up better than the number suggests.
Where I think it is weakest: the comparator does most of the work in how this gets reported, and it is not the comparator most people think they are citing.
The question I want answered is what fraction of loss being lean mass is actually normal, because the numbers quoted here range from 10% to 40% and cannot all be right. I have searched first, so if this is covered somewhere point me at it and I will read it.
Note on sourcing:
Figures above are from the primary publication rather than the press summary. If a number here disagrees with one you have, post yours and we will work out which of us is reading a secondary source.