What hung kuen originally was, a military system, a system for revolutionaries, a system for martial monks, it has all been named, but each of them requires a different type of training.
I think the students required to do a one-year of horsetraining only were disciples like you said. Testing their endurance and loyalty if they were worthy to spent the long time teaching them the entire system. Soldiers dont need the entire system, they just need the techniques to do their job. All the top fighters say the same thing, they just train 5-10 techniques they use and win fights with, it doesn't make sense to teach soldiers all of the three treasures then. The 10 killing hands were WFH's 10 techniques. Soldiers dont need anything else then their 10 killing hands. The rest of the time can be spent working on their condition, etc. instead of working through entire forms.
Just my idea on the topic, combined info without any scientific/historical evidence to back it up, of course