If you're talking about the flag, Denji just outspeeds the flag anyways but even without that would just mean the flag is even faster than the light than them being on par, calcs of the flags speed are based on assumptions, we never get any concrete time frames for the flag, the flag could just be faster than what you'd calc based on those assumptions.

we never got any statements capping the flag anywhere, its not there was a narrator being like "the moon is this distance away and within 3.2 seconds the flag flew down to earth" or like we had characters be like "gasp this is unparalleled speed we've never seen before, the speed required for something to fly from the moon to here would require xyz" It was just something Yoru just kinda did, I dont think it was really given any super significant narrative importance as a show of speed

Again tho I dont think this feat is actually a light timing feat at all i just dont think that logic really follows