Is piano an instrument? Yes. However, it can also double as an expensive table. It's the use for music, that makes it a musical instrument.
Same goes for a computer. I doubt the question was if any computer can be called an instrument. We are on a musicians forum. Computer here is a set of devices, equiped with musical software and proper input interface purposed for music creation. So again, yes. Or are we talking about office computers and pianos, guitars or violins without strings? Which you can definitely still use as an instrument btw (all of the above). Even a computer without OS. And turned off (I'm taking it to the extremes on purpose).
I would have more doubts if anyone asked me if a flute in vacuum can still be called an instrument.
- Piotr