Ehhh, that's so common in horse breeding, no biggie...
Yes, this is true in EVERY breed. I put this in another post, but just to say; in the TB registery (probably others) a horse isn't considered to be related unless they have the same dam. For Ex:
normally if 2 foals have the same sire we say they are half-brother/sister, but according to certain breeders (especially the old ones) 2 foals are only considered half-siblings if they have the same dam. They're not said to be related if they have the same sire!
Pretty silly I know.
It was also believed (not sure if it still is) that a foal didn't inherit any talent from the dam, which is why alot of TB broodmares have never actually raced/ raced bady but they still have good confo. & pedigrees.