I don't see anything terribly wrong there, to be honest. Garters really don't get lonely and don't need to be kept together. It's of course convenient that they can be kept together more readily than most other snakes, but it's not necessary for their sake. There are a couple of situations where separating the snakes is better.

However, it seems to me that the person who wrote that part was taking something that's true for most snakes and applying it to one group that it isn't entirely true for.

That thing about being too young to breed, may well be true. The larger the individual, the safer it should be for it to give birth or lay eggs. I've also read a few times that breeding them too young will stunt their growth.