Deeper or taller frame rails on the older trucks, than the new stronger rails, tells us the new trucks frame rails are made from a higher strength psi steel and probably thicker rail materials.
OP asks;
"First, is the article true? Is it only for the new 2020 Chevy/GM trucks or does it apply to other brands/years of trucks."
Yeah the article about the 2020 gm trucks is true.
No it doesn't apply to other brands or older GM trucks.
Generally 3/4 and one ton SRW trucks in the same brand/year models can have the same load carrying potential....tow ratings.
This is why we look at Fleet Ford specs website...or GMs ordering guide website...or Dodge/Rams Body Builders website that tells us the same or difference in certain year models for that brand.
What ever GM says about their trucks when advertising does not apply to a Dodge/Ram or Ford truck.