I think this quote can be true, but with what may be a big qualifier.
The qualifier, I think, is as follows (Note - the following statements I’m posing as what I believe based on evidence I have observed from my own experience and a lot of reading I have been doing on human energy systems. They may not be right, and I’m open to have my opinion changed ):
Training at intensities just below threshold preferentially develops the energy system that uses glycogen as fuel. This kind of training will prepare you very well as long as you are doing rides where you can rely on glycogen as a fuel source.
I have found that I feel great during rides/races that are three hours or less - likely when I still have plenty of glycogen on board.
For longer rides/races, threshold training may serve you less well, as it does not develop the energy system that uses fat as fuel as well as lower intensity training sessions. The training intensity where fat metabolism is at its highest in trained athletes is ~75% of FTP, and lower for less trained athletes.
When I do rides, in particular races, that go longer than 5-6 hours, I experience a noticeable drop off in power past the 5-6hr mark. Not a true bonk, as I’m usually refueling somewhat through eating, but my power output goes way down, while RPE goes up.
Up until recently, I have never really in my life done any long slow distance training, either on the bike, running or rugby. It’s all been mid-higher intensity stuff. An as a result, my fat metabolism is likely under developed - hence my experience above during long rides/races.
This is why I am trying a polarized approach for the next few months, where I replace all of my threshold/SS rides with lower intensity rides (~65-70% of FTP).
For riders who already have a big base, and good fat metabolism, then SS/threshold may give a big added boost. But for someone with my profile, who has only ever done mid-higher intensity training, and likely has an underdeveloped fat metabolism, I think SS/threshold training may not be the best approach, specifically in preparing for long rides.