Yes Windows Phone is left out, but Microsoft published a Remote Desktop modern UI app a long time ago for Windows 8/RT. That doesn't help with Windows Phone, but controlling a desktop environment from a little phone screen is kind of useless anyway. For that reason, I don't think MS is targeting phones here at all. They are targeting Remote Desktop from tablets, but it just so happens that Apple's and Google's tablet OS is also their phone OS. In the MS ecosystem that is not the case. MS phones have a dedicated OS while tablets, laptops, and desktops all share an OS.
I'm not saying this is ideal. I'm just saying that it's not as bad as it seems since controlling desktop programs from a phone is not practical anyway.