The quote should be dubito, ergo cogito, ergo sum "Since I doubt, I think; since I think I exist”, or "I doubt, therefore I think, therefore I am" to get the true meaning.
That is if you can doubt something, that means you can think, if you can think you exist. But, a computer program can express doubt, ie it can find something that doesn't make sense, express it's doubt, that is the computer can say this looks wrong, or something doesn't appear to be right, does that mean it's thinking? No, so just because you can express doubt, that doesn't mean you think, and therefore you may or may not exist.