Idk about that. 1500m probably would stagnate or get slower, but the idea that Jakob is maxing out his 800m by essentially doing mostly tempo runs and 5k paced intervals is silly. His limiting factor is is his 200/400m speed which you do not train by running aerobically.
The 800m is more anaerobic than aerobic, and becomes moreso anaerobic the faster you get at it. It doesn’t matter how many miles you log, easy or tempo, if you aren’t working speed endurance, lactate tolerance, and anaerobic (deeply sub threshold) endurance, your 800m racing is going to be suboptimal.