Earlier quoted context omitted.
My argument is that the number of people who didn't vote doesn't need to be extrapolated: apathy isn't a yes or a no vote, but something else.
Are you sure it's apathy though? There are plenty of other reasons why somebody might not make it to the polling station. (EDIT: For example the elderly who have been shown to be overwhelmingly pro-brexit could have diffculty) All polling and analysis since the result bears out that the vote was indeed representative of the population.
Another factor is complacency on the part of those that would have voted to remain: polling leading up to the referendum showed a clear remain win.
(I should point out that I'm not suggesting that the result of the referendum isn't pro-Brexit, but that the numbers don't support the case for a so-called "hard Brexit")