The question isn't so much about their phones (same as yours) or internet speeds (very damn good).
You need a good connection for sure. But for the controller, they are doing something pretty neat (I'm using it when I don't feel like pairing my Xbox controller), which is to add "custom" on-screen controls for selected games (and the list keeps growing).
For instance, for Minecraft Dungeons, it adds icons on your phone screen and you can use a virtual joystick with it, then the XCloud app transforms that into real inputs and sends it to XCloud. So for several games, no controllers are needed as they have the layer on top of it.
Here is how it looks for Minecraft, but it is custom for each game:
I'm using XCloud when I play in my bed and it is very convenient. I have a very good connection at home and I use it over WiFi but sometimes when I'm outside, I use it with 5G and it works as well (I actually have to use 5GHz Wifi at home to match the quality of the 5G. If I use a 2.4GHz network, it lags more for some reason).