Yes, I know this is likely just a repositioning flight. But being stuck at EWR with a delayed outbound flight makes you do strange things, like just stare at the Departure and Arrivals boards. There's a JFK-EWR flight showing arriving at gate 138. If you pull up flight status of UA523 on the app the heading does show "New York, NY (JFK) to New York/Newark, NJ (EWR)" but the flight is actually a 5:30pm JFK-SFO-EWR same flight number turn this afternoon.

Sad that with all the EWR entertainment options, solving this mystery is what I decided to pursue. After that, I'll try to figure out why there is a EWR-EWR flight (UA 2750) delayed waiting for crew to arrive.