Agreed, they used it to sell you a tire but ideally they wanted you to get two.
Couple of things:
- Tire size means nothing... overall actual tire size for the same "labeled tire size" are different from one manufacture to the next. This is why they do not recommend mismatching tire brands but also this is due to tire compounds, handling characteristics, tread patterns and wear.
- Your truck has differentials which are either posi traction, or open with electronic locks to engage in 4wd. The point of a Posi is to lock the wheels but also to allow the tires to rotated at different speeds (when you are going around a turn vs straight). When you turn the inner tire moves a shorter distance than the outside tire. The posi will allow the wheels to move at different speeds so you don't get noise, uneven tire wear, added stress on the system. This is not to mean that you want this "slipping" ALL THE TIME which is what you are doing essentially. This will cause premature wear of the slip mechanism which can be friction plates, cones or some other mechanism. While I have put a single new tire on worn cars that tire was the same brand and size so its just the tread wear your making up and not the differences in manufacture size and compound. If this was me I would have taken it to a different shop, plugged the tire myself and had them install it or where you are now... get another matching tire for the opposite side sooner than later. Your not going to see huge changes between manufactures but there are so many things that affect the installed DIA. Vehicle corner weight, the air pressure, tire construction, compounds, layers of steel or kevlar belts and anything that affects the sidewall stiffness. That gets compounded by the tire compounds, and tread designs which affect how the tire behaves at certain temps and road conditions.
- Your truck is not as sensitive as some of the AWD systems (Subaru's are real bad) but I would still fix it with replacing with a matched tire on the other side.
- I'm not sure how the size of tire will affect your wheel speed sensors, or if the systems are even sensitive enough to measure that difference. I would suspect not as they have to account for wear but also would have tripped the ABS or traction control systems by now.