I'm 6'2"/315 and ride a bike that's rated for 265 (I think, RAD City). Have had ZERO weight related issues (or any other issues) with about 2000 miles of use over the last 3 years. I would qualify that with the fact my bike is used on pavement and hard packed trails, and ridden in a sane manner by a senior. Further, and maybe more importantly, this bike's original 750w direct drive hub has been replaced by a MUCH more powerful 1000+ watt geared hub, potentially stressing the bike's components by an additional factor of at least 2.
Point being, I wouldn't hesitate putting a 280lb rider on a bike rated for 265. Not for a second.... -Al