I've never heard of tires alone being the cause for a warranty denial.
Think about it this way, though. Larger tires put much less load/stress on the driveline than a 7,000LB trailer. Towing a trailer doesn't void the warranty, larger tires shouldn't impact it either (unless the tires literally damaged the truck because they were making contact).
Though larger tires while towing at max rated capacity is outside the design limits. Also larger tires put different stresses on the drivetrain and suspension components than towing does.
While the OP may not ever tow a trailer(ever). The dealership would not know this. For all they know, the OP towed at max GVWR while running the larger tires.
It’s part of the same reason why an aftermarket calibration is such a big deal to GM. They can not verify the conditions the vehicle has been through between when it was factory stock and when it returned for warranty repair.
Typically you will not have a warranty issue due to a small change in tire size. But, if they want to make a point about you using parts that are not similar or equivalent to factory spec for your truck based on vin number, they can.
I modify my vehicles and do not expect to have a warranty issue. However, I know that it is a possibility. If you make the choice to do any modifications to your vehicle that are out of factory specifications, then accept the possibility of being your own warranty station.
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