Quote jarvis12345="jarvis12345"momentum has to stop before you can call a double movement doesnt it? if so a try with BoD was the right call!!'"
That is correct.
The rule is that a player who has forward motion is not tackled at the point where his ball carrying arm hits the ground, but at the point where his motion stops.
The laws are not clear cut on this point, and there are two different sections that address the same area. However, firstly, it was definitely NOT the case that he was stopped short, but reached over the line to ground the ball. His movement, i.e. the roll, was what took him over the line.
Once over the line, he is permitted to ground the ball.
Quote jarvis12345="Laws of the game"
Try – How scored
3. A try is scored when:–
Sliding try
(c) a tackled player’s momentum carries him into the
opponents’ in-goal where he grounds the ball
even if the ball has first touched the ground in the
field of play but provided that when the ball
crosses the goal line the player is not in touch or
touch in-goal or on or over the dead ball line.'"
The debate should therefore be whether he was stopped short of the line, or whether his momentum got him over the line.
If he had been stopped a foot before the line, and then twisted and reached over to ground, then a definite penalty, but he didn't.
Watched at normal speed there didn't seem to be any obvious stop in his forward motion, and so the decision seems perfectly reasonable, if you take the time to read the actual Laws.