TL; DR OpenBSD acted rationally if they'd prefer to go it alone, which seems to be their culture. To their credit, it's worked pretty well so far. But you can't have your cake and eat it too. If they prefer a mad scramble after public disclosure, they'll get it. But they shouldn't get early notice from responsible researchers.
I don't believe that embargo is healthy or responsible! If anything its a monopolising factor.
Which doesn't make a difference if OpenBSD still gets their patch out at the same time as everyone else. Unlike other vendors, it doesn't take OpenBSD four months to go from vulnerability notification to patch release, if you look at previous disclosure timelines they typically have a patch out in days.