Why? He did not think to look for aluminum single strand wire because the home was "too new" to have single strand aluminum wiring.
Although he was correct about the age of the home (it was built long after aluminum wiring was commonly used) he missed a major safety defect that is very expensive to cure.
Someone must have found an old coil or two around when the addition was added.
What does aluminum wiring look like? How would most home owner's or Realtors know the home had aluminum wiring? If the home has BX (metal exterior wiring) it does not have aluminum wiring.
I have never seen aluminum wiring in BX and have never heard of it being made.
There is a tin coated copper wire in BX that was very common.
It is NOT a hazard at all.
If the home has Romex wire (plastic or cloth covered) look at the writing on the wire.
If it says CU or copper it is not aluminum.
If it says AL or aluminum it is aluminum.
Removing electric panel covers is NOT a job for those who do not know how to do it.
There is enough power inside the average electric panel to kill you and a few others.
If you are in doubt hire and pay for a professional to determine the type wire in a home, it is well worth it.