I thought it was a try and was rather surprised it wasn't given (but putting my Warriors bias on I was happy it was denied), as I've said with those decisions going to the defending team it is reward a bad defensive miss read which I think Koopu, Witty and Swann all did. The orginal gap I think was made because Koopu rushed out of the line (do that at your own peril I guess), then the decoy runs through without actually making contact with Witty and Swann and they were going in the same direction the decoy runner came from (so he's sucked them in). Space there, ball receiver gets it and runs through.
In the Raiders v Warriors match, didn't one of the commentators say if the ball is passed beind the decoy runner it's not an obsutrction? Only if the player with the ball runs behind that decoy then it's obstruction? If that's the simple version then yes a try it should have been.
But bad decisions like this have happened all year, and Manly let it bother them too much, even when they were denied that try they still had every chance to score points and win the game but didn't.