He is right about the diversity.
I bought my original mini sebastopols from Dave. He is the only one I ever found that breeds them, so I introduced several other bloodlines to them and now have 6th generation minis I'm working with. So even that is far enough away from the original bloodlines that they can be used together in a breeding program if needed.
It's important, as he also mentioned, to find other breeders since each works towards their own lines. With a breed that's hard to find, you need the diversity from other breeders to help keep the breed going (as long as it doesn't offset your own breeding goals for you flock). Otherwise you have to rely on crossbreeding for a certain number of genetics to get the diversity, which can set your breeding flocks back many years. It's better and easier to simply switch with breeders who have the same goals