Got an answer from Microsoft Support with a Premium Ticket, so updating the answer here as well if someone else is wondering the same:
"We have feedback from engineering about the status ‘working elsewhere’ set as ‘0’ in Graph API. This is currently by design as older Outlook clients use the same value – 0 for the status ‘free’ and ‘working elsewhere’. In order to support all Outlook versions the status of ‘Working Elsewhere’ and ‘Free’ are set to ‘0’ in Graph API as this is the value that is set for meetings.
We do not have a timeline on when this might be changed as we are still supporting older versions of Outlook that use this method.
The documentation will be updated by engineering to reflect this."