I am skeptical of relying on this and wonder if the system condition ought to exist at all. The question is "what defines mobile?" Most people would say a tablet qualifies, but then what if you add a keyboard? Then a mouse? Now you have a laptop, which most people would say qualify as a desktop type system. What about a large phone with a mouse and keyboard attached? Where do you draw the line? If you force a touch-only UI on these devices, aren't you ignoring the fact they can type and use a mouse?
IIRC the condition basically checks if it's a "mobile OS", i.e. Android, iOS, Windows Phone (which now is just part of Windows 10, making the distinction even blurrier). But note those devices can also run in what is essentially a desktop mode.