At some point, some basic knowledge would have been enough, nowadays the more knowledge, the better. You can be an UX designer for many types of design, but you need to know the available possibilities for the requirements. If mobile, you'll need to be aware about the differences between the different devices and operating systems, controls, DPI, sizes, behaviors, etc. Please note that I'm NOT saying an UX designer has to be able to program an app, but she needs to be aware of the possibilities.
The bottom line is: you can design an experience if you know the reach of that experience and the elements and components that create that experience. Otherwise, you'll be creating something incomplete, incapable of reaching the max potential (because... hey, that little thingy you didn't even know it exists.... well, it could have saved months, or make the app more powerful,or logical, or whatever). Or even worse: you would be creating things that can't be done, or doing them would mean a cost that would render the project impossible.
Of course, this doesn't only apply to mobile, it applies to each and every branch of UX. For example, an UX designer could design an spaceship control to go to Mars, but she doesn't have any idea about how to design a website with a reasonable UX. And I certainly wouldn't ask the best website UX designer in the world to build the usability of my spaceship unless she spent quite some time working with the NASA!!
And if in doubt, let's take an example: take a look to iOS HGI guidelines. Would you REALLY want someone that ignores that and adds months in terms of revision or development simply because the UX designer ignores it all about the subject she has to design? REALLY? I bet you don't! And if so, I doubt you'll last long in your company. After all, you made them lose thousands of dollars "just because"
It's like any discipline. You have specialists, and specialists knows their specialties. As simple as that.