I feel like if I saw something just called "learn unix", I'd expect it to be teaching me how to use it generically, not implement it. Adding "OS" makes it clear to me that it's a dedicated OS for the purpose of learning (although it still wasn't clear to me that the goal was implement it so if anything I'd argue that the title is missing context, but the name of the OS isn't redundant).
So your argument is the first usage of a word gets exclusive rights to it? Firstly, that's not how human languages work. Secondly, this would invalidate the Unix claim to "nix" as it's been a word for hundreds of years prior to Unix being invented.
It is, but in IT context the association was strong, while Unixes decline and most of the systems with derived naming are historic. But anybody with a background in sysadmin for more than 10 years probably would still have the association. In ten years Linux will probably the only one remaining with the ux-naming (and MacOS X with the single X, which also serves as ten, following MacOS 9)