It is official.
See the link below.
https://medium.com/@eosnewyork/the-eos-mainnet-launch-security-software-testing-efd3f6c40f45
This article was written by EOS New York. EOS New York is the top 21 Block Producer on the EOS Mainnet Blockchain.
I asked EOS New York about this content via Twitter. EOS New York responded as follows.
Block on isn't official anymore. The EOS Mainnet repo is.