If you hit the player after he is past the 1st down mark, fail to take him down, and he escapes backward taking the ball ball behind the sticks, then tackled with the ball remaining behind the mark...that would not be a 1st down, it would be placed where then ball was when the non-hand/foot body part touches the ground.
In this case, he was retreating in an attempt to escape, was not forced to the ground yet by a defender, then his momentum and the D-player tackling him took the ball behind the goal line, then his knee touched down as the ball remained behind the goal line.
That is a safety.