Isn't it all relative and cumulative? For elite runners who have worked their way up to being able to sustain 100-120 mpw, they aren't as fatigued doing their easier stuff because they're used to the stimulus of that much consistent mileage. So at a certain point they are no longer receiving the stimulus of just plodding around running slowly because their aerobic systems are well-tuned that the only thing they can change is going faster or further increasing mileage or altering recovery doing workouts. They're already probably at altitude most of the year as well.
For 18 min 5K runners, they probably don't have nearly as much aerobic conditioning and physical conditioning (as in the pounding and stress on joints). But there's a big difference between them running 7:20 miles for "easy" runs vs. 8 mins or even slower because that's both more taxing on them aerobically and physically in terms of their joints experiencing stress. There's more variables than just what running "easy" is as a function of their race pace.
They would just benefit more from going slower and letting their legs adjust to the stress of repetitive pounding over long periods of time rather than go faster on these easy days. Combine that with going too fast on hard days + going too fast on easy days and this will probably lead them to get injured and lose the consistency of just playing it safe and going slower on less important days, rather than getting the short term benefit of more aerobic conditioning but injuring their joints/ligaments. Jakob seems to always be healthy because they build up long, long term consistency and continuous aerobic development rather than attempting to force workout paces downward like many of the Americans did for years.