When Fry and the kid are skateboarding through LA, they hit a light pole. When the dust clears, the light pole is gone.
Even though in "Mother's Day" it's clearly stated that the technology behind the wheel is lost in time (as most of things hover in the year 3000), in L.A. you have skateboards and normal cars driving on the highway.
Fry mentions brains flying through space before they ever meet brains flying though space. Even if they knew the planets in a later episode were being destroyed at that time (which isn't mentioned), he wouldn't know what caused the destruction at this point.