Different advice for different goals. If your goal is to survive, you want your vitals to be protected. If you want others to survive, might as well use your muscle and fat to absorb some of the kinetic energy.

The idea is obviously for everyone to lay flat with their legs facing the grenade, not just one person. If there’s just you and one other person just force them to the ground. If you live in a dangerous place explain it to them beforehand and maybe even practice.

that then your legs

Well, not one THEN the other. It’s a single blast and it’s too quick to change positions.