That's because what he is showing as magnesium is actually die cast also known as pot metal, magnesium is much lighter than aluminum.

And die cast is heavier than both aluminum and magnesium.
Yeppers!! Surprisingly, for as much as I used to hear about "pot metal" when I was younger, I don't really come across it much working my various finds. Maybe its cheaper to use straight cast alum for a lot of things then dinking with zinc and a stew pot of whatever...

https://en.wikipedia.org/wiki/Pot_metal