I wouldn't recommend extending that argument to far.
The weight has very little to do with the structural rigidity / integrity or load carrying characteristics of a given design.
The fact that a design is old has nothing to do with structural rigidity / integrity or load carrying characteristics of a given design.
In point of fact the opposite is true....
Garry Fisher and his friends discovered this in the early 80's when he started the mountain bike craze....Old frame designs weren't designed for the loads he was asking them to accept.
Engineering has come a long ways in 85 years.
Tig welded frames using modern alloys and taper'd tubing are able to support the loads you describe while a lugged and brazed frame may not.......
This by no means describes many low buck box store bikes.
Your blanket statement :
A bicycle frame most are 10 lbs or less in total will not support 300 plus lbs as implied from more than a busload of people claiming they will that's all.
You have kind of painted yourself in to a corner here by not using words like ( some , or many ) You imply that all bike frames are unable to carry sed loads.
You sir, are wrong............