Hrm, I don't think they lie, I think sometimes they are just lacking in info. Really, with us bigger guys it's hard to predict what's going to happen. The energy it takes to move a 150 pounder up a hill is significantly less than it takes to move one of us up a hill. As such, you're putting more power into the wheel. And there's more strain over bumps, etc etc.
That being said, what did he sell you for a new wheel? How many spokes? I'd suggest trying a different shop, find a local one with a very good reputation. Being in Pittsburgh I'd imagine it shouldn't be too terribly hard to find a second opinon.