I was under the impression that the point of the domestic flights on the 77W was so the intl FA's/flight deck crew could be familiarized with the aircraft so when intl service started things would go without issues. Guess not if this is their first time on the plane!
When the 77Ws first came they were sCO... only swapped to sUA after the third frame was delivered. And IIRC SFO-HKG is crewed by the HKG base so pretty unlikely that any of the FAs would have worked an SFO-EWR trip before that. I think.