This was true at PC launch, when horses had levels, you trained horses individually and they were limited to a sum of 50 level ups. At that time, buying a 42k horse made sense since you would get 10 additional "levels" for them having 10 points advantage at level 1.
Nowadays, all horses are same and have no influence on how far the rider can train (since it's not the horse that you are training, they are called "riding lessons" for a reason).
TL/DR: old read. no difference between 10k and 42k horse.