So what?
You referred to it as Cameron's referendum, the Uk Parliament supported it with a massive majority.
David Cameron's prime motivation was to put a lid on the ERG. He was convinced that any referendum would support the status quo. This view was the mainstream view within the House of Commons. He even permitted a simple majority to determine the outcome.
Both main parties were certain that a national vote would support Remain (as did the LibDems). Hence the overwhelming result of the free vote.
I think the political establishment voted it through to deal with growing support for UKIP, who had won the EU elections in 2014.
Greens, LIbDems, Labour at various times all have supported a referendum, not because they wanted to leave but because it was an issue with the electorate.
The ERG may or may not have been Cameron's motivation, but the reason we had the referendum and have now left is because of the electorate.