How do we know that it never happened? Well, absolutely no verifiable fact in the story stands up to close scrutiny.
St Columba and his fellow Irish monks had already brought Christianity to Scotland, two centuries before this battle occurred. A century after that, the Synod of Whitby had seen the Roman Catholic Church traditions win out over the homegrown Celtic Christianity.
The cult of St Columba, which was huge at the time, had to be quickly eroded. It was suggested then that St Andrew was a more appropriate saint to patronize the Scots.
In 738, twelve years before the alleged battle, Acca, Bishop of Hexham, in Northumbria, had to flee his See. He arrived in Scottish Cennrígmonaid and found refuge there. He had brought with him bone relics, which were purported to have been St Andrew's.
It boosted religious pilgrimages to the Abbey of Cennrígmonaid greatly. So much so that the area was already being called St Andrews by the time Óengus and Athelstan clashed at the site.
And about that battle. There's no record of any Athelstan in Northumbria during that period, let alone one invading Scotland. And Óengus II didn't take the throne until 820, only seventy years after he was supposed to have been taking his people into war.
Apart from that, it's all true. Every word.
However, let's be fair. We haven't looked at Óengus I yet, and he was ruling at the right time. Moreover, he was embroiled with wars against the Northumbrians. But there's still no Athelstan and the St Andrews Sacrophagus (pictured above) was already in the Abbey by 747.
In short, there's a lot of mixing of half-truths and legends; but it was true when it needed to be.
That was when Robert the Bruce delivered the Declaration of Arbroath in 1320. The long association of St Andrew with Scotland was used, in a bid to convince the Pope to recognize Scottish independence.
It was around this time when writers, in Scotland and in France, created the story in the form that I recounted above. It helped consolidate the case and cement the foundation tale that's still told today.
Though, as patron saint and early king respectively, I think that St Andrew and Óengus II would have been happy to oblige!