This is wholly incorrect. If you truly believe this, then you haven't learned enough. While some of the GUI tools, the package management, and minuté file system structure details are different, the back ends and overall structure of the file system is the same. If you use GUI tools for system configurations, you will notice a lot of differences. However, if you edit the configurations yourself, you'll find it is _very_ similar. The point being, anything you learn on one distro will carry over in one form or another.

If you haven't looked at the page, everything gone over in the sets of tutorials will all translate over not only within Linux distros, but any POSIX-compliant system. The tutorials go over POSIX CLI commands.