I didn't watch #loseastonein21days because that sounds like quite a dysfunctional way of encouraging weight loss, but based on my own experience when you eat below your BMR too often, you lose a lot of muscle and water and not much fat.

— Red Sky At Night (@redskyatnight) August 12, 2020

It depends on what you eat and how you do it.

Do it stupidly, and yes, this will indeed happen. How to avoid this is to eat lots of protein, having eating “windows” of six hours or so, and do resistance training during this time.

All of those tend to promote muscle retention. You’ll still lose a little muscle, but might actually get stronger (depending on level of current fitness).

People do things stupidly, and are surprised when stupid things result. Always mystifies me.