Yeah, that doesn't track. I could understand "new" tires being a couple years old because of the whole international shipping/port issues that have arisen. But, 3-4? Sorry, but that's pre-Covid. Which makes it before the factory and port shutdowns. Nope. I get it, the ports are backlogged now and we will be dealing with that for awhile because like a lot of things the ports were running at max capacity before the shutdowns to maximize profit. And if you're already at max capacity clearing a backlog isn't fast. But it is definitely being used as an excuse to screw people over too, and it seems like that's what's going on here.
And what's legal is a whole different story. Maybe it's legal to sell somebody 5 year old tires as new, but if no one will repair them past the 7 year mark it sure as heck isn't right.
Sounds like you need a new tire shop. If nothing else, they certainly don't seem too concerned about keeping you happy or doing a good job.
I think you missed the part where he said he only puts like 6k miles a year on this particular truck, they were going to age out before they wore out from the beginning. Which makes the age a lot more important. I don't stress out too much over timed out tires as long as they still look good, but even if they look good and are 7+ most tire shops won't even touch them now. So your ability to go past 7 years is entirely dependent on not needing any tire repairs.