Five-hour startup work day is impossible

A German startup that develops websites, apps and e-commerce platforms, has a five-hour day work policy. Employees come to work at 8am (apparently 8am is not even strictly enforced) and leave at 1pm. That's an hour after noon! Apparently productivity is kept up by cutting out the fat - cell phones are turned off, company emails are checked only twice a day and meetings are kept to 15 mins. The company was profitable in 2018, its first full year in business. A San Diego startup experimented with five-hour work days and after two years limited it to the summer months only. This is all reported in the WSJ. I've been a founder and a CEO. I've represented startups when I was an I.P. attorney, and later consulted for startups on the business side. I just don't see how it's possible to do a 5-hour day. I am a devoted husband, father and son, and am just green with envy if this is really working for some startups. I have a heck of a time cutting out work after 10 hrs a day. I doubt this 5-hour work day thing is a publicity stunt by these startups. It seems genuine. Comments?