Physiology is a fickle pickle. Yes, while Strength training really doesn't impede distance running the opposite can't really be said. Studies have shown distance running impedes strength and power performance. The amount varies and is likely individual. IE: a trained endurance athlete could do more aerobic activity without it impeding strength/power development.
Long Jump, High Jump, Shot Put, Discus, Javelin, Pole Vault....6 events.....all very power focused. You need to generate high amounts of force to propel yourself or an object.
100m, 110m H....are all about speed
400m, 1500m....these are the "distance races".
Jumps, Hurdles, Throws involve a ton of technique and even the sprints involve starting technique.
8 events are under 10-15s of max effort and most are instantaneous max efforts.
Without looking up any sort of decathlon training my guess is they would train like a short sprinter who also does power events. Like a bulked up Carl Lewis who was a sprinter/LJ. That's probably the base of their training along with the various technique work and gym work. From there you address weaknesses. Do you need more sprint training? More power training? The 400 and 1500 are probably the last two events to worry about. You also need long days. Little mini decathlons or similar exercise programming so you train your body for efforts spread out sporadically over two days.
Alan