It is a 5 person engineering team, so I’d give them the benefit of the doubt. But more importantly, where are the salary ranges? You are required by law in California to post salary ranges.
Why is the team size more relevant than the equity share when evaluating fairness of 1 day weekends? Since usually full-time workers aren't compensated for overtime, and startup pay is usually low, while founders and investors take the lion's share of an eventual payoff
5 person engineering team is the total size of engineering presence. They mention they don’t want managers till they grow to 20-30 engineers, which makes me believe this is a very small startup and not an established company. It is very usual for employees at companies that size to work more than just 9-5 5 days a week, usually in anticipation of a big eventual payoff.