Because he stopped Kizaru’s light beams.
Not matter how near he is to whitebeard “which is nowhere seen or stated in the manga panel” he wouldn’t have the time to reach and stop those light beams unless he is FTL.
I found this narutoforum thread where they say Marco is not blue fire but is plasma which can also move at light speed.
So is Marco plasma? Not blue fire which was stated to him by the author himself because he has phoenix appearances and that phoenixes are made of “fire”.