So if you're life currently involves only long days filled with multitasking (probably very common in some walks of life) the solution involves shorter days and more focused time, whereas for others it might be the other way around.
Whatever the truth is, I highly doubt humans are optimised specifically for exactly eight (or six) hours of focused work every day. I suspect we are helped by some variation, including the occasional 14 hour multitasking day.
Also wouldn't it make quite a difference on what time scale you are multi-tasking? If it's on the time scale of days between context switches or hours/minutes.
Unfortunately it’s difficult to test that hypothesis. And it’s survivor bias to pick Elon Musk.
Working 12-14 hours a day on a couple of things is more in line with deep work, which is actually very productive for alot of people.
If I had to guess, it's that these studies aren't sensitive to the outliers. Imagine telling e.g. Elon Musk "multitasking and long hours are bad for your productivity".
I've been keeping very detailed data on my personal productivity for years, while taking all of the consensus wisdom ("long hours = bad", "multitasking = bad") to heart. What I've instead found is that long hours (literal 14+ hour days) makes me more consistently productive.
Multitasking also has a strong synergistic component to it that I suspect doesn't show up in these narrow studies.* If multitasking across different projects leads to insights that shave years off of your respective project plans, but otherwise makes you slower locally, it's still more than worth it. Again using Elon as an example: Tesla and SpaceX have been made mutually stronger by insights being cross applied the two enterprises.
* To be fair I'm speaking more of multitasking at the project level, not at the minute-to-minute level.