Hi the ball has been kick into the dead ball area the attacking player slids in, but goes passed the ball then reaches back and touches the ball down ... Try given ? although all his body is outside the dead ball area ... isn't he out of play ?
If the ball has gone past the dead ball line, then the ball is dead and we look to a restart.
What The Rugby Ref thinks you are saying is that the ball was lying in the in-goal area. In that case any player may press down on the ball for a try or a touchdown.
From your description the attacking player did indeed press down on the ball, so the question is, can he do this from outside the in-goal area.
The answer is yes he can and the try was good, provided he only pressed down on the ball and didn't pick it up.
Law 22.4 Other Ways to Score a Try
22.4(g) Player in touch or touch-in-goal. If an attacking player is in touch or touch-in-goal, the player can score a try by grounding the ball in the opponents' in-goal, provided the player is not carrying the ball.
Thanks for the question.
The Rugby Ref