Another way to look at this is that it is based on Microsoft's internal distributed systems infrastructure and thus will never be open-sourced for the same reas

Another way to look at this is that it is based on Microsoft's internal distributed systems infrastructure and thus will never be open-sourced for the same reason Google will never open-source Spanner, Megastore or Colossus. Having battle-tested internal systems to build on is nice, but it means the DocumentDB code is probably nearly impossible to run outside of Microsoft.