Just an anecdote, but: my productivity definitely took a huge hit when I reduced my hours to 4days/week. All of the "working harder/more efficiently/more focused" doesn't help if you already worked hard/efficiently/focused. If you have a job with a lot of slack/downtime/waiting time... Sure. But sometimes, working 20% less means that 20% less work gets done.
Most people are nowhere close to working fully focused 8 hours a day, and may also benefit from more time to recharge. Others benefit less. The average probably matters more for the company (also shorter work hours will improve retention, or allow you to pay lower wages to compete etc).
Personally I'd think of the extra day as rent paid on not having to hire and onboard someone new.
Also productivity does not go down, because not everyone is doing it. ( so people have motivation to not get fired )
In normal conditions, expected would be some drop in productivity but bigger gain in retention.