I don't quite get it yet. I'll phrase it as a hypothetical - lets say two cyclists are following the same 2 x 20 minute intervals with a five minute easy pace in between, but rider #1 trains a or near FPT whereas rider #2 other trains at 20 watts over FPT. My thinking is that rider #1 is improving tempo riding but rider #2 not only is improving tempo but maximal aerobic power as well. All else being equal, would'nt rider #2 be faster?
Perhaps rider 2 would be faster if they both just did that single workout. But in reality rider one is likely to do longer and or more intervals per session and be ready to do another session tomorrow and the following day before needing an easier day where rider 2 is unlikely to repeat a set of intervals above threshold on consecutive days. Even if rider 2 can pull off a two or three day block above threshold they're very unlikely to do that week in and week out.
A wise cyclist once said "fitness is an integral", IOW it's not so much what you do on a given training day that leads to greater fitness. It's what you do throughout your training weeks, months, and years that defines your fitness. It's easy to suggest really hard programs that look great on paper but lead to burnout, slow recovery, mental overload or just plain quitting the sport.
But sure in theory if you could do all your L4 efforts above FTP and could recover quickly enough to do them frequently in your training week and could do it for as long as the guy working below FTP most of the time - then yes you would expect greater adaptation. But a lot of folks have found that difficult to actually do in practice.
Go ahead and try it and see how well you bounce back from consecutive days above threshold or longer efforts above threshold. You'll likely find that you'll end up doing less quality work per session and less per week than you will if you back off just a bit. That's the whole concept of Sweet Spot Training, work at levels that are sufficiently hard to encourage the desired adaptations but sufficiently easy that you can rack up a lot of sustained quality time.
That doesn't mean we don't work above threshold, even during L4 work, we do. It's just that we don't expect to do that every session or as standard SST/L4 work. As the racing season approaches I tend to do one day per week where I really go after best efforts for my 20 or 30 minute intervals, when things click those are often above FTP. But the following day I'll do longer efforts backed off to ~90-94% of FTP and the following day it will be longer yet and SST work. There's nothing wrong with some intensity if it works into your training week in a sustainable way. Just don't think you can shortcut the process by bumping up intensity for every session. Many folks have tried it and most weren't very sucessful punishing themselves that much day in and day out.