If you remain perfectly still, you’ll stop sinking at your waist
I'm not sure you understand buoyancy. Floating objects float, they don't sink no matter how much thrashing about. And while quicksand is denser than water, it's still not dense enough to float a human at their waist, it's actually around their chest.
Nobody dies from drowning in quicksand. Ever. It's always dehydration/fatigue.
Science, real science like Elon is describing, happens when you write stuff down. "Published science" is where the glamor is but that's, quite obviously, not what Elon was talking about.
So sad to see bitter people lash out at the successful. (projection is also a classic trait)