Scotman:
The ball also becomes dead when it crosses the boundary, or when the umpire calls 'Over'. Bell was (apparently) under the impression that both of those things had happened but has since accepted that neither of them had and that his dismissal was completely within the rules. (i.e. the umpires were correct in their decision, after review, to give him 'Out').
TheCorbyLoon's question however looks at a later stage in the proceedings and examines whether the umpires were entitled to accept a withdrawal of the appeal (allowing them to reverse the 'Out' decision) after Bell had left the field of play.
I fully agree with the underlying assumption in that question. (i.e. the rules, strictly interpreted, did not allow the appeal to be withdrawn and the 'Out' decision to be overturned).
However, like Lborobrewer, I assume that the umpires used the 'get out' clause of assuming that Bell had only left the field of play because it was the start of the tea interval, and not because he accepted that he was out.
Chris