I'd presume that they'd always been optimized that way. If you make a prime telephoto lens as good as it can be wide open, it will also be as good as it can be stopped down. Making it better wide open doesn't make it worse stopped down.
The only reason to make a lens better when stopped down is that it's cheaper. Making a lens that sharp wide open needs extra corrections and that costs money either via extra elements, more expensive optical glass or increased fabrication costs due to more complex element shapes and/or better quality control of surface accuracy and polish.
With wide zooms you can probably make tradeoffs of wideopen sharpness for field uniformity and better performance over some focal length ranges, but you don't need to make such compromises with long telephoto prime lenses.
So my guess is that there is nothing to that rumor. If The Canon telephoto lenses have gotten better wide open, it's because of advances in design and materials, not a change in design philosophy. Their prices have certainly gone up!