For clarity, and while the HN seems to imply that, that is not what this decision was actually about.
It was about the specific requirement that disputes be handled by binding arbitration. The circuit court was actually clear they weren't making decisions about the facts of the case, precisely because the arbitrator gets to make those calls.
Now, sure, that can mean "you lose" in practice, depending on the claim and the arbiter. And in this specific situation it's a death knell for the plaintiffs, because this was an emerging class action suite looking for a big payout.
But no, the 9th circuit has not found that companies have the ability to enforce "arbitrary terms of service" via a TOS update email. They only made a call on this particular term update, and they were clear that they did so because it does not represent an actual change to the service terms (only to the dispute process).