Using my own HS and the years around my time there (in the late 80s/early 90s dead era in high school running) as an example...I lived in a small suburb of Seattle, not really attached to the city, in a county with about 50000 people. Our high school had about 1200 people if you count the 9th graders who actually went to the junior high and had different (nonexistent) coaches.
We got a new coach who sort of knew what he was doing compared to others at the time but was far from great. We ran 30-40 miles a week and blasted intervals just like every dummy in the late 80s. We also ran very little in the winter (3 times a week). Also ran club track (same coach) in the summer, extending the season into July rather than doing any endurance base for XC. Then, at least during my time, we had a totally bad coach for XC who didn't know anything (more like the junior high coaches - he later lost the XC job to our track coach).
My class was the beginning of our program developing decent runners and winning conference championships. One of my teammates ran 4:32/9:48 and later went on to be a college all american and top 10 in the US marathoner, he also ran 4:20 the second year out of high school while self coached and focused on marathon training (and not running at all the first year out of high school). I ran 4:21 on less than 30mpw as a senior and not running at all in the winter. Another teammate ran 4:22 and 9:09 (was top 20 in the USA). A couple years later they had another guy come straight off soccer and run 1:56/4:23 with about 20 miles a week running and not running at all in the winter or summer. He went on to run 4:11 in college but probably had sub 4 talent, just never any coaching, never ran over 25 miles a week. Another of his class in high school ran 4:22 and 9:20ish, while a couple other guys ran 4:24, 4:27, one of whom ran 4:00 for 1500m the first year in college. Finally, a slightly younger guy ran 4:18 and 9:11 a couple years later.
Nobody in that group had coaching in the 9th grade. When I ran junior high track as a 9th grader I doubt we ran more than 30 miles in the whole season and I recall doing 3 or 4 interval workouts if you can call 3 x 200 or 2 x 400 a workout.
So in the span of about 5-6 years, a small 3 year school of 900 not in a major city had 8 guys in the ballpark of 4:20 with a decent but not great coach (he was a school record holder at a PAC 10 school so he knew what it took to be good but strangely we did not train very hard - that was the trend in the late 80s). Mind you this was in the worst era of US running and without today's shoes.
I think people underestimate the amount of talent that is just laying around not getting developed. A coach like Brosnan only needs a school with a couple thousand kids and some buy in. With his reputation already established, the buy in is much easier. During my time Pat Tyson was the Brosnan and he continually turned out low 9:00 guys and state champions. He did have the Davis family but I think that just proves that if you're a good coach and you find a talented athlete that has brothers, you'll have a good chance to bring through several big stars from a family because they have similar genetics and they'll follow in their brothers' footsteps. It should be more surprising if a successful coach doesn't have a set of brothers who were good.
I do not believe for one second that a coach could go their whole career without obtaining one guy capable of running sub 4:20, certainly not in a reasonably large school, hence Brosnan's idea that 4:20 is not that fast.