you know too much to feign ignorance.
8th grade is a bit early to be concerned with is my kid future d1 and where are the cutoffs. just enjoy school and track and see where he fits in soph year in HS. and the sort of 405 times you are lobbing around cannot be just trained and work rated. past a point god either hits you with a thunderbolt or not. getting out the whip hand on your kid to chase 405 is insane. i knew kids who rewrote our junior high record book then plateaued at like 425/high 16s.
you're also missing the "400" point. to me what he's getting at is the top HS distance kids as you keep progressing usually have solid varsity level 400 times in addition to their distance event times. high 40s/low low 50s. natural fast athletes. you can't teach that. you can't mileage that.
like at my HS the best 2 milers would be like 52-53 if you time trialed them in 400. won't be on mile relay or open quarter, but they could win a JV race and not finish last in a varsity heat. if you have that level speed but also the distance motor, it's easier to turn quick splits without immediately blowing up, as you aren't redlining running 60ish splits. a kid who runs 60 flat and tries to run 2 flat 800 or 4 flat mile pace would detonate, he's running full speed just to stay with a lap of that pace. he's cooked. so it matters how far you can get under that 60 and it be controlled not a sprint.
and there's no way you know that in 8th grade.