Yesterday I tried this for my self. Here is what happened:
3 games which I uninstalled were done successfully through the app.
1 small game which I haven't launched led me to windows unistall page.
So, it is now possible, in most cases entirely through the app.
The game "Bejeweled which I bought very cheaply on a wim, never launched or played
had to be uninstalled via windows (the app linked directly). I ended up not uninstalling it as
it takes up barely no space and it was just a test.
The other three successful, fully within app uninstalls were big games. The way to do it was easy.
In game library click the three dots on game icon. From there you can verify files and the option to UNINSTALL is there.
As said already it worked fine for three games. I needed to free up space so it functioned as a spring clean, but I did it due to
this thread.
That's my experience. Make of it what you will. I summary, games CAN be uninstalled entirely through the app only.
With the caveat that some games may send you windows programs list where it can be installed via windows. One extra click.
Curious to know if it works or not for others. Can only speculate as to why one of them linked to windows (Windows 10) but I have
no idea really so will leave it at that.
It works.