Minecraft pocket edition has very little in common with minecraft on PC at the end of the day.
Pocket edition primarily consists of a C library. They then use a wrapper to expose the libraries features on android, iOS and raspbian independently. The raspbian version has even further modifications.
Minecraft on PC is java.
Ultimately pocket edition is just a glorified minecraft clone from the same developers. Shares no similarities code wise. Highly unlikely that it could ever link between the original java version, the PE version or the xbox version. They never claim anywhere that it can do that either.
They do claim that the hardware controls are for Xperia Play, they make no claim that it is compatible with any other gamepad.
The Shield controls and of course external gamepads are exposed to android as Human Interface Devices and you can use the standard android library to interface with them. The Xperia Play controls are not exposed as an HID for some reason, some of the regular buttons can be treated as an HID (with really weird mappings actually, think circle is mapped to Alt Key plus the android back button or something bizarre) but the thumb"sticks" cant and sony give an actual android lib which covers the regular buttons too, most developers just use that, including mojang for some reason.
Honestly, I think they need to get their fingers out of their asses and add HID input with proper assignments and joystick support. There are quite a few android devices out there with hardware keyboards etc,. Hell, sisters netbook came dual booted with android but has no touchscreen, that would be even more unplayable than the Shield for minecraft without 3rd party apps (and no, cant just switch over to windows and play MC there, without the optifine mod it gets 2 fps, with it gets 3, unplayable, increase java priority and the system cant keep up and you get input lag anyway, all drivers are updated)