Technically most things we call “Linux” aren’t even Linux, they’re distributions that use Linux as a base. Android forks their own Linux kernel and then shovels on a whole bunch of non-Linuxy stuff on top of it and then locks it all down so that you can’t make any changes.
So what people mean when they say they want a true Linux phone is that they want an OS on their phone that behaves more like most Linux distributions (where you are in control, and can even install a different kernel if you wanted).
As for the forking question, being downstream of Android means A) undoing everything they did it lock it down and B) constantly having to do that moving forward as they do everything they can to stymie you at every turn.