If you would have clicked that link, you would have seen that the post is from Feb 2018. The video in the post is John Mueller from Google in December 2017 saying the same thing that Matt Cutts…all after RankBrain was added to the algo.
Google STILL claims they treat them the same.
While my experiences aren’t in line with their statement, you cannot say “Google interprets content off your primary domain as a poor user experience” when Google is publicly saying the exact opposite.