IIRC, Snyder said that Barry could punch at light speed, but was too afraid to. I don't read comics but isn't there also something about Flash needing to go light speed to time travel on that treadmill? That said, I think it's very likely that it's legit. If you're asking about how it scales, I question if he was going his top speed against Superman (due to fear) but then again, I don't think he would be that scared of Clark if Barry could go faster.
I take it back guys dceu flash Can BFR or Erase Quicksilver as a child it was shown that he can time travel and travel between universes in the crisis on infinite earth he spoke with Cw flash in the speed force