He may have had an old one that wasn’t quite expired at the time he got a new one, and just kept it. I keep all of my old passports, and a lot of people do. I like to see my old hairstyles, LOL!
I have heard also that some people whose job requires them to travel to Muslim countries in the Middle East have a separate passport or a removable page in their passport if they have traveled to Israel, since many Muslim countries will not admit anyone who has an Israeli stamp in their passport.
Actually it was probably filled up
When I went to Israel, they gave me a stamp on a separate piece of paper to put in my passport.
When you get a new passport, the Government has you mail in the old one. After you get the new one, they mail back the old one, which is marked as expired so it cannot be used.