[Roodgorf]

I'm pretty sure they're just referring to the immediate feedback of being able to practice by reading with one hand and playing with the other. You generally wouldn't be able to play a whole piece that way, but in comparison to most other instruments you don't have to do as much memorization up front.

You're right though, there are definitely plenty of other instruments that would work for. Trumpet for example would be an even better candidate I imagine.

[jcranmer]

You still need two hands to play the trumpet: one to hold the instrument up, the other to operate the valves. If you used only one hand to play and hold it, effectively the entire weight of the trumpet would have to be borne on your thumb.

[TylerE]

I would strongly disagree. Trumpet music is MUCH simpler than piano music, as piano music is highly polyphonic (many notes at the same time), while wind instruments are only one note at a time (excluding a few VERY advanced techniques that are rare).