H1st0ry is not wrong at all.
Tanks are the largest units on the field. They are both front and center taking abuse. If you are Ana shooting down range and the shot goes through a full health target, you might have hit a squishy hero behind it. Which can happen with smaller targets who naturally avoid damage from being smaller.
With double tanks, that kind of thing no longer happens as much. You are more than likely going to have some tank that’s taken damage, your shot goes into them, with double tanks and double defensive abilities up possibly, more of your shots and CC are simply negated.
You have literally nothing else to do except heal tanks. If you do stop trying to heal them, their lower hp leads to them dying more quickly. Supports heal more often. Blizzard has even stated in interviews that 6v6 leads to more healing supports as whole.
DPS are going to hvae a harder time trying to get access to backlines to shoot other DPS or supports because of the same thing. Double tanks someone likely is going to get in your way, block an angle, stand in front of some choke point, etc. So much more of the action goes through tanks with 2 of them on each side.
It slows the game down dramatically too.