Except the devs specifically say this isn't a fork (Score 5, Informative)

But why bother reading #1 in the FAQ? 1. Is this a fork of the Mambo project? No, it is a rebranding effort that will continue to run largely on the existing codebase. Work is continuing on the project by the same team that has developed Mambo as you know it today. Therefore we see it as continuing development rather than a 'fork'.