While you're right, the MTOW of a B-26 is around 17 tons, the 767 is 150-200 tons.
So there is a factor of around 10 between them, so if the 767 flies 3 times as fast - which it doesn't, the B-26 cruises at more than 0.35 Mach at close to sea level, and the 767 is not supersonic - that means that the factor from the speed can't be more than about 3 squared, so 9.
So the factors from the weight and the speed are roughly equal IMO.
I mean the modern skyscraper is definitely built very different these days.
The world trade center used hollow exterior support so they could avoid having support columns interrupting the floor plans and large central support columns but you can see what happens when the exterior support gets damaged and heat causes sag from the weight.
Advanced techniques usually mean less material and faster build times.
You know what was even more solid? A huge pile of rocks in the shape of a pyramid.
Different/less fuel I imagine. The problem with WTC was the fires kept burning which weakened the steel enough for it to collapse under its own weight.
Edit: Admittedly, I read the headline as "B-52" but I think the comment stands.