

Great explation. One exception:
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.
As others have pointed out the issues, I will answer a different question.
It would be cool to have a backup heart. It sits unused and quite until needed. Clearly some issues with experiencing the same conditions as the first heart. If it were biological, then it would weaken without use.
But a small pump to survive a heart attack would be nice.