I think it's a referendum on the terms of the Brexit deal not Brexit. The Greens have already proposed a similar idea as part of their progressive alliance proposals. They accept that the public have voted for Brexit but there was no discussion of what that actually meant in terms of any policy etc. So the electorate should be consulted. But I've no idea how that would work in a referendum context (general election maybe) as I can't imagine the EU giving us a range of options to vote on! In theory though there is the potential for a progressive Brexit and er... a not so progressive Brexit. So the electorate should have a say in the debate.
I'm in no way endorsing Owen Smith here by the way - I've no idea who he is.