I think that in the origins of St Andrew and the Saltire there are a number of things all conflated.

The story of the saltire in the sky seen by Oengus before Athelstanesford seems to have remarkable similarities with the story of Constantine and a similar phenomenon before the Battle of the Milvian Bridge (in hoc signo vinces) that ended up with him adopting the Chi-Rho as his badge.

There's the possibility that Andrew (Aindrea in Gaelic) might indicate the male correspondence of the same Celtic divinity that Boadicea sacrificed the Roman women of Colchester to (Andrasta - Andrastos): i.e. 'Victory'. It wouldn't be the first pre-Christian divinity that found its way into Christian Scotland: one thinks of St Bride, for example.

And I'm not sure whether the Scots of Dal Riada, would have been too impressed by a Pictish story whether from Fib or Fortrenn.

The Scots, pre-Christian, had been "sun worshippers" (i.e. Lugh Lamfada), using the sun colours red and gold, and the lion device (hence the Red Lion arms), or might it actually be the Lynx as the word is close to the Greek lungks (transliterated) meaning "light" and the Celts were big on correspondences. The Picts, apparently, had been "moon worshippers" which is supposed to account for the prevelance of blue and silver/white in eastern Scottish devices (such as with the Murrays).

My guess is that Kenneth Mac Alpine - supposedly a Scot, but I'd bet that he came from the Brythonic Strathclyde area (Alpin isn't a Scot name, but a Welsh one) - when he was proclaimed the King of the Picts and the Scots adopted the Pictish saltire with its narrative as a unifying move (he also palmed a fake "Stone of Destiny" off on the Picts of Fib, which became the Stone of Scone: the actual fragment of the Tara Stone is supposedly still at Loch Finlaggan on Islay).

No matter; the Scots of whatever origins nowadays keep the Saltire and St Andrew as their flag and patron saint, and rally around them.