which, by nessesity, means the parents aren't American either.
As the speaker didn't witness the birth of their own parents, the speaker simply does not know if they are Americans. It is not a joke about immigrants. As you correct state, lt is a joke about an unwillingness to believe what one did not personally witness.
Your point remains, but often those are magnetic loops, not weight.
Everyone else, start noticing cuts in the roads near traffic lights. Stop in the weird shapes in the middle of your lane.