In my view, a trip in vain generally does not include a trip where you complete the outbound portion of your itinerary on schedule.
Had there been an active shooter situation at PHX which delayed your takeoff from your origin or from a connection point, and while delayed, it became apparent that your trip would be in vain, then flying you home and refunding the trip would be expected.
In your situation, the airline fulfilled its obligation and got you to PHX on schedule (or close-enough). The lockdown of the airport isn't all that different from a closed freeway as soon as you leave the airport. Had you encountered an overturned semi that prevented you from getting to your meeting in Mesa or Scottsdale or even downtown Phoenix, the airline probably isn't going to entertain arguments that your trip should qualify for a refund.
I've had a few situations on AA over the years where mechanical or weather issues arose after I departed a spoke and once at the hub, the delays ensured that my trip would be in vain, and AA flew me back to my point of origin and allowed me to reschedule the trip/refund to a voucher. But never because local law enforcement held me hostage at my arrival (stopover) airport and prevented me from leaving the airport in time for a meeting.