The study[1] was based on starting from 100% Office randomizing the amount of days people worked from home and was measured by email quantity, quality, and manager performance reviews. A better wording would be "employees send more and better emails when working from home 60-77% of the time". To fully understand the problem you need to study going from WFH to RTO and productivity. Multiple studies have shown the benefits of 100% WFH[2]. Hybrid and RTO is being pushed primarily by management not employees.

[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4068741 [2] https://www.apollotechnical.com/working-from-home-productivi...