We choose the definition of sentience so that it includes us. There's no reason to believe that an alien species might not do the same, and that we might not make the cut. Hell, some of the worst racist rhetoric on the planet involves seeing other humans as non-sentient.
The whole point is that we're blind to our own biases, as fish are blind to water. That includes our bias towards physical matter over simulated matter. (It's funny how the notion of a simulated universe unsettles people and destabilizes their narratives, despite the fact that discovering it would change precisely nothing about the subjective reality of our universe.)
See also: http://terrybison.com/page6/page6.html