It didn’t require a supermajority, but it did in fact deliver one - 67% “Yes” versus 33% “No”, a margin of 34%, or two-to-one, in favour of remaining in the EU. Plus, the “Yes” vote won in all four parts of the UK.
But, yes, the result is up for revision, and indeed the UK is right now in the process of revising it. But the question was regarded as settled for a very long time because of the size and spread of the “Yes” majority.
The referendum is something of a constitutional novelty in the UK, and they haven’t really worked out what it is for or what role it plays. Some Brexiters seem genuinely to imagine that a referendum is some kind of show-stopper; that it silences democratic review and reconsideration of any question for an unspecified but lengthy period. To support this view they point to the effect of the 1975 referendum, but I think they miss the points that (a) this was a political effect, not a legal or constitutional one, and (b) it produced this effect not because “Yes” won, but because it won by such a massive and widely-spread majority. Opponents were free to continmue democratic opposition to EU membership; they just recognised that such a campaign would find no traction.
The 2016 referendum was touted as settling the question for another generation. This always seemed to me an obviously bogus promise. Politiclans are simply not in a position to decree that a referendum will settle any question. The whole point of a referendum is that the people make a decision, not the politicians, and if the people don’t give a decisive, substantial, widely spread majority to one side or the other - if what the referendum discloses is a population more or less evenly divided between two sides of a polarising question - then the question is not settled for a generation, regardless of what rash promises any proponent of the referendum may have made to that effect.
What the 2016 referendum has shown is that the UK is deeply divided over EU membership, and that a “winner takes all” mentality is not going to heal that divide, or even paper it over successfully. And don’t imagine that another referendum, even if it does 52-48 the other way, will be any more successful in “settling” this question. The UK needs to think about how to function as a state, and succed as a society, while living with this divide.