they can say it, but they can't void the warranty unless they PROVE it. Now I'm sure there will be plenty of dealers that will say they can't honor the warranty, but you can either press them until they do (not worth the hassle, IMO) or find a dealer that will honor the warranty.
My boss just put 35x12.50 Nitto Mud Grapplers on his stock 2018 Rubicon. We both did a lot of research before ordering and ordered the 1.75" spacers from Spidertrax just in case, plus he kinda wanted the look of pushing the wheels out further. When the tire shop installed the tires they checked...