I imagine it would also be possible to write some XSL to transform one to the other then.

No I don't know the details of either format.

Regardless of what they use, even if it's unmodified software from another source, they are still distributing it. The GPL requires you to do two things if you distribute software covered by it:

1. Include the complete text of the license
2. Include a written offer to obtain the complete source code (or include the complete source code, or a location under the distributor's direct control where the user can download the source code)

And then of course GPLv3 adds the requirement that the distributor must provide a way for the user to install and run a different version of the software on the device.

The important part here is that the distributor of the binaries must also distribute the sources, you aren't allowed to just point to another party for that.