Look closer. The sandbags cancelled a split second before they saw the Pathfinders. This is caught well on the clip from Brosras account.
And even if we pretend he did see them: how can you react in 0.1 seconds ingame when coh2 servers have more input delay than that and when humans also have a reaction time?
Agree with this. It's not possible to stop building that fast in-game. If you hit S or whatever hotkey you are using once you see the enemy approaching your own unit takes quite some time to actually stop and get going due to animation and ping. In this clip it definitely looks like he stopped before actually having the unit appear out of FOW.