When you say you can "use" the latest Qt with all these operating systems, I believe what you mean is that you can run programs written using Qt on those systems. Other than desktop Linux, none of those OSs actually use Qt internally for anything (and only some distributions of Linux use Qt).

I would guess that, if you really wanted to, you could build an executable on Qt 5.5 and run it on a Jolla; you simply wouldn't be able to take advantage of Sailfish's GUI. You'd need to include your own UI graphics libraries into your app, much the way Qt works on Android. (Gah, I really, really hate the way Qt works on Android; I've been pulling my hair out trying to create a UI for my apps that looks at all reasonable. Creating a reasonable UI using Sailfish is so much easier...)