I suspect when he said “they’re like new” he meant that they were only partially worn and functionally there was no significant deterioration.
Tires have “wear bars” molded in. When the spaces between the treads get close to the wear bars it’s time to replace them. When you think about it, tires are one of the car’s most critical safety items and as such are really, really cheap when you consider they can mean the difference between life and death. When in doubt, change them out.
For a good primer visit www.carbibles.com.