Not a tyre expert by any means but I believe that is the thought process behind it, also when the older tyre is at the limit (1.6mm) to be replaced the new one will still have +4.6mm remaining so what would you do then? if you just replace the older one then you’d have to go back to the tyre shop again another time when the other wears out and vice versa. I change in pairs for that reason.
Agreed it can be argued that it’s slightly over the top though.
The point is you save yourself £125 and although as you say will need to be changed again in the future, but he might only do 5,000 miles a year and could last another 3 years or more. He will no doubt replace with the same make and size.
Are all your 4 tyres above 5mm, are you at risk of aquaplaning, do you replace tyres well before the wear markers ?
I think I agree with your last sentence though.