As noted, Cena appeared in an opening segment with The Usos, RAW Tag Team Champions The Revival, and WWE Hall of Famers Rikishi and D-Von Dudley. This led to The Usos defeating The Revival in a non-title tag team match, but Cena left the ring area before the match when the show went to commercial.
Cena was recently in England filming with "Fast & Furious 9" movie with Vin Diesel. There was speculation on his RAW Reunion appearance after he did an interview with a Florida TV program on Friday. He has a home not far from the Amalie Arena in Tampa, which was mentioned during the RAW promo.
On a related note, there was a lot of talk about how Cena mentioned The Usos' arrests and mugshot photos. Cena made the references when trading rap lyrics with The Usos, right after they knocked him for leaving WWE for Hollywood. As noted during Wrestling Observer Radio, those lines were most likely pre-approved ahead of time.
As seen below, The Usos took to Twitter and responded to the arrests mention from Cena.
They wrote, "Expected nothing less from 'The Doctor of Thuganomics'."
Expected nothing less from "The Doctor of Thuganomics".?? https://t.co/HT1O3Y9moB— The Usos (@WWEUsos) July 23, 2019