How to train, particularly for elite athletes with lots of time, has been generally known for the last 50 years. Mostly easy with lots of volume. One or two intense sessions per week that are hard, but not crazy. Every so often, really go deep.

None of this stuff is new, or revolutionary. It's been largely settled science since the 70s, or perhaps even earlier. The things that are actually debated are the details.

The big benefit of easy (or "zone 2") is that you can do a lot of it and still recover. Volume matters. No reasonable person thinks 1 hour of easy work is the same as 1 hour of threshold though. A time crunched athlete is obviously going to cut the easy stuff first.