The wheels assembly rotates around the motor axle and acts as suspension. This approach however introduces extra friction on the motor axle when turning. But because the robot is tiny, this doesn't matter greatly. As you can see in the first picture, the wheels assembly is made out of plastic, which acts as a low friction bushing and the bushing part is wide enough. In the next pictures, the wheels assembly is made out of some fiberglass or something similar, probably from a PCB board without the copper. The bushing part is narrow, which creates problems when turning.
I really like the tightness of the board and the miniaturisation, but still using regular parts, not SMD parts. The motors are driven directly by the PIC microcontroller, not through an Bridge circuit. Pretty hard to find motors that will not draw more than 20MA. I think these motors can be found in old Mac floppy drives.