No, not true. If you research components, you will see that there will be weight limits on things like carbon forks and bars (some carbon steerer tubes are not built for big folks - AlphaQ had or has one). Wheels sets -- I think I remember one from Velocity that had a weight limit. The stuff is out there. Even stock bikes are built with the assumption that the rider will fall within a certain weight for his or her height. The bike may not technically have a manufacter's restriction but it sure won't perform or last as well as it would for someone within the expected limits. It is always worth asking if you are concerned. Ask the shop about the durability of the bike for heavier rides. Wheels will generally be the first thing affected. Some frames have the same issue. Maybe no weight restriction but just aren't happy with a heavier rider. Some frames don't like lightweight riders and beat the crap out of them (this is where aluminum frames got their reputation as the first tubing had no compliancy). Really lightweight steel and Ti tubing can get flexy in large sizes. So, you have to consider performance as well as warranty.
Wider tires can make a comfier ride for anyone.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.