That's true, I saw somewhere that he ran 40MPW in high school, not sure if that was XC or track though. He ran 1:47 in high school, still amazing, but he already has dropped 4 seconds on this new training. Maybe he just has the years of 40MPW as a base so he can do this lower mileage training without losing anything. Still, 40MPW isn't much of a base for someone on his level. He could just be an anomaly, but he is doing so well I doubt it.
I see this a lot. He was doing xc in hs and it helped his running. Now for the first time in his life he skips xc and drops a ton of time. Short term, this is great, long term, he's going to probably need to keep up with the xc(or mileage in the off season). It's just like any runner, remember how much Cam Levins' time dropped when Salazar "pulled in on the reigns"? How much has he improved since then? I know the 800 is very different than a 5k, but it's something I think about.
I won't try to act like I know what's best for Brazier, but I always wonder why 800 runners are afraid of mileage in the off season.