No, it's slightly faster to touch the top. Mario spends the same amount of time on the actual flagpole, but he gets more acceleration if he slides from the top.
He's talking about doing the flagpole glitch in 8-1 like Kosmic does in earlier levels.
But the flagpole glitch only saves like 15-20 frames IIRC and a perfect 8-1 has you 21 frames away from the next frame rule so executing the flagpole glitch in 8-1 would save 0.000 seconds.
Kosmic would save time if he did the flagpole glitch in 8-3 though, but I guess he's not comfortable with that glitch.