Of course but I think about it more as the software it's running being the instrument rather than the computer itself. I guess @Rctec organ point could counteract that argument since each different sound on the organ isn't a different instrument in and of itself.
If 3 people are all playing completely different sounds using completely different UI devices (keyboard, drum pads, motion sensors, etc.) but it's all running off of 1 computer is there only 1 "instrument" present?
Anything that makes a sound can be an instrument. But it has to actually make the sound not just play back the sound.
I guess there is a grey area where, if you add a significant amount of post processing, that's controllable, in real time, to a sound then it is arguable that you are "making" the sound not playing back a recorded sound.