I do think there is one clear benefit as measured in the article - employee retention. It's an extremely strong incentive to stay with the current company, and domain knowledge gained over decades really is a competitive advantage if your employees commonly have it.
What remains to be seen though are if the efficiency gains are good enough to justify less hours. Are employees more productive? That's the question that remains to be answered or objectively measured here. Less burnout and better mental health means higher quality work for sure, but is output as a whole better with a 4 day work week versus a 5 day work week? That's what shareholders will care about more than anything else.