A physical taurus embedded in our 3D universe, i.e. a donut, cannot be flat. Which may be the confusion.
But video game spaces are neither logically embedded or restricted by our 3D space. All its physics, including its spacial topology and geometry, flat or otherwise, were completely up to the game designers.
And the Asteroid game designers chose a flat space.
• Two parallel line segments stay parallel, no matter how long you extend them. Or how many times they wrap around the Asteroid space. (They also never intersect, but can overlap, just as in any flat 2D space.)
• Any number and distance of moves in any two perpendicular directions, are commutative. I.e. you can change the order of the moves, and you will still end up at the same spot.
• The three angles of any triangle in Asteroid space adds up to 180 degrees.
None of which applies to the surface of a donut.