Just in case anyone else nerd-sniped themselves this morning... if things fall at the same rate in a vacuum, regardless of their mass, why does it matter if one side of a die is heavier than the rest? I didn't know, and I had to look it up.

It's correct that a biased die will fall without bias. But when it hits the surface and starts tumbling, it tends to rotate around the center of gravity, which will be closer to the heavy side, and the die wants to end up in the orientation with the "lowest gravitational potential energy." If that term isn't part of your lexicon, then think of a Weebil toy.

I’m not sure how much of an effect it has, but dice aren’t rolled in a vacuum. There is buoyancy from the air that can roll the dice heavy side down. Which is what the saline water test is testing.

Perhaps also worth noting that you generally shake the die before releasing it. Thus even if you drop it straight down through a vacuum, you would have done the center-of-mass-impacted-tumbling in your hands first.