My dad worked for startups on-and-off and he definitely was productive for a lot more than 8 hours per day. He does agree that for truly novel work there's a limit, but for every novel problem to solve, there's dozens[1] of reported bugs to investigate, so there's plenty of work to fit between work on new things.

1: That number seems small today, but I got the sense that bug-discovery by customers at least was limited by the low number of customers that a b2b startup had in the 80s.