it is but he was wrong to a point. when you take away genetic diversity to much that is what causes low fertility, low birth rate. linebreeding is great to bring out prepotency in your line and to fix traits. but while it will fix good traits it will lock in bad ones as well. like i said earlier if the defect it locks in is a simple recessive trait then it can easily be removed but if the defect is polygenic then not so much. when you are breeding you can only work with the genes that are there and line breeding limits those genes

example say you are breeding your line and out comes a dog like chinaman and you start to line breed to get another chinaman this is great and you will get some good dogs but none of them can be genetically better than chinaman because those are the genes you are working with. now when you outcross you bring in new genes to the mix and they will line up in different ways and you can make the genes you had better than chinaman. What some people do though is either line breed to much or outcross to much neither is good for the dogs you need a happy medium