Yeah we do run across tires that are off by a fair bit sometimes. Some brands are more inconsistent than others. Anything more than .25″ and you can feel it on track. The kart will pull to one side.
Just like a sprint car, you run a larger outside tire to help the car turn left on an oval. Though karts work a bit differently, that’s the principle that’s being applied. If you have a bigger tire, putting it on the RIGHT side will help the kart turn left. I wouldn’t adjust air pressures to make up circumference differences either. That’s a pretty integral part of chassis tuning. That’d be like running an extra torsion bar to help straighten your bent frame.
What makes you so skeptical of this guy telling you to put the larger tire on the outside? What rationale do you have for putting it on the inside? Would you not also be upsetting the balance of the kart? And how would it affect caster or camber? Just trying to follow your train of thought.