Is "sīc" used as "yes" when answering a question common in Classical Latin?
Terence (and relevant here too is Plautus, to a lesser extent also Petronius and Apuleius) give us the best 'literary' evidence for colloquial Latin at any period, so I wouldn't be too worried about Snodv's 'pre-classical' reservation. ita can be used in the same way.Charlton Lewis tells us that sic was used alone as "yes" colloquially. He cites Terence, but I would say Terence is pre-classical.
This is how it's used in Sardinian, it's used when I give a complete answer to a question.was often used with clarifying words