First, the fumble was ruled correctly, so we should not complain!
I do however not like the rule. The goal of the rule is to prevent offenses to take advantage of a fumble late in the game. This makes sense. I my opinion however there is no reason why the offense shouldn't be able to bring the ball back to the line of scrimmage! You shouldn't be able to advance the ball past the line scrimmage though. In this case, the most favourable result for the offense would be just a lost play without the loss of yards. Therefore there is no way to take advantage of a fumble late in the game.
I agree, the ruling was correct but IMO the rule most likely wasn't written with the intent of a late fumble in your own territory resulting in pretty much an automatic safety.
I would write the rule the same way as you that the ball will be spotted where it is downed, with no better result than getting it back at the LOS.
However this would have made no difference in this case as Lacy did not make it back into the field of play before being tackled.