Microsoftwrighta16 - this version was made available through Office Insider Fast, which requires opt-in and is very clearly identified as:
Best for Insiders who want to use the very earliest builds to identify issues and provide feedback about new features still in development. Insider is ideal for those who don't mind the bit of risk involved in using unsupported builds.
It's not, as you wrote "released to the public," quite the contrary - this is a way for the Office Mac development to get critical feedback on the implementation of new design, UI/UX, features. I posted something about this new version on twitter yesterday, and several members of the Dev team reached out personally and encouraged me to share my impressions, said it is very valuable for them to hear from real-world (and in my case, very long time) Mac Office users and helps them prioritize which issues to work on.
