There are a few possible solutions:
- Greninja was holding a Quick Claw
- Reuniclus was holding a Full Incense or Lagging Tail
- Reuniclus acquired the ability Stall
PX Edit: Greninja might have also used Shadow Sneak, which bypasses speed via priority. Also Trick Room might have expired and you didn't notice.
No PX, he clearly said that Trick Room was in effect and Greninja used a move without priority. If he hadn't then I would've added those in.
EDIT: I have searched several Pokemon websites and all of them said the same thing: the only things which override Trick Room are priority and priority-affecting moves and abilities: Stall, Quick Claw, Lagging Tail, and Full Incense. As such, the only possibilities are that you missed something or that this is some weird undocumented bug. Case closed.