Anyone who has actually fasted knows this is all nonsense. Sure, you can lose muscle mass — that's always true of going into a caloric deficit, which is the only way to lose weight.
I don't see anyone here questioning that you lose muscle, but questioning the idea that confining your eating to a narrow window (or a single meal) is bad.
I'm not aware of any evidence to support that. Fasting patterns have been studied, and I'm not aware that any studies on fasting have shown the effect you describe, but I've not followed the research for some years.
In terms of anecdotal evidence, I have regularly measured body fat percentage during diet periods, and I do see steep drops in body fat percentage when I've done intermittent fasting with the vast bulk of my intake coming in a single meal (2k calories plus during one period). As mentioned elsewhere I now prefer 4-5 days of drastic cuts and eating normally a few days a weak, and without confining it to a narrow period, but I also did IF for many months while at my peak of lifting both in a deficit and not, after having previously done slight deficit dieting in the past, and could compare the difference, and I didn't see the slightest hint of the effect you're claiming.
