I looked at the video again. If you look it in slow motion, you can see that there is another man crouching in the back. Although the man in the front might not be visible to the tank driver, the one behind should be and has no shield.
Also, just before the impact the tank fires the water cannon in the exact direction of the two. Might be a coincidence or a proof they saw them.
But even in the event they did not see them, you just don't drive a tank against a barrier where you don't know if there are people or not.