In my opinion what hes saying is true, but has to be taken with a grain of salt. The choice of the word “pointless” is a little harsh but i understand what he means. They are only derivatives that dont accomplish anything that the distro they forked cant accomplish, ergo they are useless because you could make Ubuntu on debian.
As for why debian and arch are the best, they are the two most well established community maintained distros. That means they have the most people working on them, the most support out there on the internet when you encounter issues, they tend to be the most stable, AND they have no corporate backing which can be seen as “evil” by some people (like Chris in this video).