It was briefly free OnDemand for a few days. I think this is either a glitch or Comcast changing something to force people to start paying more money.
It's still listed as available on demand on HBO's website: https://www.hbo.com/schedule-search-results?productIds=618573 and in the past all currently airing HBO movies and TV shows have been availabe in SD with subscription at no additional cost along with available older films and past TV series. It's also still free on HBO Go.
I just checked and the Free SD version has been re-added to On Demand. It's available until 3/17. At least in my region. Hopefully that means it's available for everyone.
King Arthur: Legend of the Sword still has not been re-added as a free option.