You lost muscle because you lost around 1.54% of body weight per week, which is way too aggressive. The maximum recommended amount for losing weight while retaining muscle is around 1%. You will also most likely experience a weight rebound.
I thought that this was not yet substantiated in human trials? There's a plausible mechanism and, I think a mouse trial, right?
It's also not borderline impossible to maintain the majority of your muscle mass, but it depends on how you eat and train. We don't know enough about the person above's diet, training, current body composition, etc. to say anything for certain.