I think it's just the skewing of the whole time and intensity inverse relationship.
A hobbyist racer like myself has plenty of time to recover and have pretty much all the intensity per week you can. Because you have almost infinite time off the bike to recover compared to the pro.
The pro, has all the time in the world on the bike. So, instead of having a 9-5.......they spend that time on the non-intensity time AND the intense time. Since it is lower intensity, you can eat more of it.
I'm pretty sure recent studies show that SS and other higher intensities still cause more of those changes......but at the cost of reduced time. If they've already maxed the intense time they can spend, they're left maximizing the less intense time to build up more.
It's a horrible analogy, but in golf you can only hit so many hard shots on the driving range per day. Like, maybe 200 or so? That's your higher intensity. But, you can chip and putt until you're bored out of your skull. That's your z1/z2 work. Low intensity. Aderall, golfers, look it up. It's a doping thing. They can both focus during events and during mundane training for longer. Like putting and chipping for hours on end.