I would actually strongly disagree with that.
Firstly, it would be difficult to see how a transmission cooler could cause an engine to fail. I'm sure that Nissan would prefer the car to remain standard, but given that Nissan have themselves effectively declared that there is a problem with oil temps by way of the warranty restrictions, if an owner fits something that only helps to resolve this known problem and in no way affects the engine, then I'd be very surprised if they would bounce a claim, or if a court would find in their favour if they did.
In comparison to the chance of a warranty being voided for Cobb, exhaust or y-pipe (which in my personal opinion I would say are 'almost certain', 'possible' and 'very remote' in turn), the effect and chance of this happening for an oil-cooler is tiny by comparison.