Table 6-4 (Page 55) is very clear ...
IF THE KICK BECOMES DEAD IN K’S END ZONE
If the force is:
1. The kick or any other new force by K.
Then the ruling is:
1. Safety, if the kick is out of bounds or K has
possession, including when the ball is loose
Since K's kick is what put the ball in the endzone, K better get it out of there.