Is it in the next sentence about muscles reaching fatigue toward the end of a 10k run, and that fatigued muscles are where adaptation occurs.

It depends on what kind of adaptation you want.

I personally view cardio as just a means to improve my cardiovascular system, not muscles. So I just do boring steady-state (where I can easily rap out a song or sing Bob Dylan's lyrics).

If I want increased mitochondria and the size of the muscle cells, then nothing beats an even more efficient method of strength training. Just 5 minutes of work (1 minute per set) of lifting a heavy weight for 5 times. But even here there's concept of RPE where you don't accumulate as much fatigue as you might think.

I first did strength training and my resting heart beat dropped from 75 to 60 after about 8 months. After that I started doing steady-state cardio of 30 minutes on rest days and it's now at 55 after 3 years.

You could run say, a short warm up jog if 800m, a couple of warm up sprints of 200m, and then a few sprints at MAX effort. 60s break after each run. About 2km total. Your legs and lungs will be severely taxed. If they aren’t, you ran too slow.

You can easily take a 5km run and increase the effectiveness: go faster.

Fitness runners always fixate on distance. No speed, power or agility training.

I think there is a difference between running short bouts of Max speed sprinting versus running long periods at a lower effort. It is the difference between zone 2 and zone 4/5 running, taxing different systems (aerobic vs anaerobic).

I am not saying there is a need for speed, power, agility training as well, but I think it is achieving different things than long, slower runs.