It has two sides. They’re curved, and it doesn’t stay on the curve part, so you can effectively use it as a d4, but it’s still only two-sided.

Sort of like how you can flip a Mobius strip like a coin and it will land one of two ways, but it still only has one side.

I like how you’d be rolling two d10’s, and then completely ignoring one of them.

Good DMs anyway.