I'm sorry but I don't really see what you're getting at. You seem to understand about what changing the front sprocket does with the speed/mileage, you've even done it on your bike, so it's not like he's trying to be deceiving (if he did the sprocket change right away).
So what if he didn't put the speedohealer in? At least he never did it and is able to say "it's this off" rather than "I put in a speedohealer around halfway and I estimate it's around #### off". If you think he's just flat out lying, take a peak at the sprocket, or if you were sold on buying the bike but weary of that, see If he has a recept or If he got it done somewhere (possibly when he bought it) go and talk to the dealership. If you're paying big money to buy something online like that, I'd hope you'd do SOME kind of looking into it.
Idk this just doesn't seem that off to me. The guy explains it well and seems more legit than a lot of posts I see of people explaining shit like that.
If the person selling is on this site, best of luck!