While I agree water pump problems on the 3.5L are relatively rare, they do happen. I think the earlier models (2007-2009) have a slightly higher failure rate though.
Either way, the dealer probably recognizes changing the water pump is a major PIA and they might be trying to duck out of having to do the job. So keep that in mind.
If you want you can run your own tests to confirm. Drive your car for 20+ minutes to get it fully warmed up, then restart it in ETM
(hold the reset button while starting the car, keep holding it until the cluster display changes). Keep pressing reset until you get to the coolant temp display. Now just sit there and let the engine idle for 10-20 minutes. As the temp climbs, you might hear the cooling fans kick in. The cooling fans should be able to keep the temp below 105ºC, but if you see it start climbing past 107ºC and the fans are running full blast it's a good indication you have a cooling issue. If you reach 110ºC you need to abort the test and get the car moving up to 40 MPH so you can cool it off!
Continuing with the test, now hold the engine at 2000 RPM
in park and watch what happens to the temperature (record if it goes up or down first). If the coolant temp drops more than about 10ºC it's a good indication that the water pump isn't moving enough coolant.
You can exit ETM
by shutting the car off or by holding reset again to exit.
Record the numbers and report back with what you find. If you want you can record a video of the test for us or as evidence.
If you do have a water pump problem, you will know for sure come summer!