The message may be heard, but putting it into practice is the more difficult task in my opinion. Changing a whole company's (either big or small) policy regarding meetings/working hours/etc... is quite a challenge
Still, the new book's cover may attract an audience that is not familiar with DHH/Jason/Basecamp's opinions on work/life balance.
Yes, company dynamics are really complex and this book will only have an effect if the people reading it are the company leaders. But they're also reading "10 habits of successful CEOs" articles that tell them to wake up at 4am to send emails!
Ultimately correlation isn't causation. Working 40 hours doesn't guarantee success by itself and neither does waking up at 4am.
Still, the new book's cover may attract an audience that is not familiar with DHH/Jason/Basecamp's opinions on work/life balance.