He didn't.
This bullet and several others were not fired at NORMAL speed expected from say, an AR15 and it's basic ammo.
The 'sniper(s)' did not want to get caught so they use a special load which is deadly quiet. It also is not quite as accurate. The bullet itself is larger and weighs more than a standard bullet and with less of a powder load and/or use of a slower burning powder does not propel the bullet as fast.
End result is that if it hits it's target, it is just as deadly, but it is unlikely anyone will hear it.
A subsonic bullet would leave an image of itself less than 2 inches long at that shutter speed. Can a subsonic bullet leave a condensation trail? Subsonic jets do, but their engines are turning fuel into water. A big fat bullet without an engine? Don’t know.