Just because something was posted on Trainorders does not necessarily make it true. I definitely don't consider Trainorders, in general, to be a reputable source, unless something is posted by about 4 certain people, two of them travel agents with direct access to Arrow. You will note in that Trainorders thread that when you asked the original poster about the schedule, he never named his source, only saying that it was not yet posted on the Amtrak sites.
With that said, I did find a note in the past hour in the March newsletter for the Rail Passenger Association of California (RailPAC, of which I am an absentee, across-the-country member) which states that it was announced at a February 19th board meeting of the Capitol Corridor Joint Powers Authority that a 12th train is expected to start in April. News like that I do believe.