Interviewed at a startup where the CTO did live coding while answering Slack. Normal or a warning sign?

He pasted my half-finished function into the chat and asked for a 'quick review.' Then a DM popped up and he went quiet for three minutes. To be fair, he was friendly and the problem itself was good. I'm just not sure what to make of how the round felt. Has anyone worked somewhere like that and had it turn out fine?