That's like saying a donut only has one side.

The linked die seems similar to this: https://cults3d.com/en/3d-model/game/d1-one-sided-die which seems adjacent to a Möbius strip but kinda isn't because the loop is not made of a two sided flat strip. https://wikipedia.org/wiki/M%C3%B6bius_strip

Might be an Umbilic torus: https://wikipedia.org/wiki/Umbilic_torus

The word side is unclear.