I'm not surprised the 2 XD isn't listed. That Roku is so old it's no longer a supported model. But it should be there for the Express I would have thought. But Apple is the one that controls which device it's available for. There have been a couple of other threads about the Apple TV app that list the supported Roku devices, so I suggest doing a quick search for them and check for your Express. Like I said, I have no doubt the 2 XD is not supported.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.