The scene was lowland Scotland, and the narrator was speaking about the coming of English to this area, beginning in the 5th Century. English arrived with the invading Angles. But then, he said that English replaced the Gaelic that had been previously spoken in that area. This is simply wrong. Gaelic was never the local speech of lowland Scotland. Gaelic came in from the Isles to the west at the same time that Old English was coming in from the south. The language spoken in the Scottish lowlands was Brythonic, which is more or less synonymous with Old Welsh.
The British (not Scottish) kingdom based on Edinburgh (Dun Eidyn) was Y Gododdin, which some take to be the British (Brythonic) way of pronouncing the old tribal name which the Romans took down as the Votadini. Meanwhile, over by Glasgow, British (Brythonic) kingdoms such as Strathclyde were also part of Yr Hen Ogledd, "the Old North." The Britons known to the Romans -- from either side of The Wall (Hadrian's, that is) -- were all one culture and spoke dialects of the same language which still lingers in Wales. North of the Firth of Forth, the Picts held the rest of what became Scotland.
Eventually, the expanding Scots (that is, the Irish, from the island called by the Romans Scotia) of Dalriada united with the Picts and formed the kingdom of Alba -- what we call Scotland. That kingdom then began to absorb the peoples whose British tongue had lost out to English. Gaelic was the official, royal language, but nobody would have dreamed of making everybody learn it. Pictish died out and at least among the power elite was replaced by Gaelic -- except in Caithness, where invading Norsemen left a language called Norn that lasted several centuries. Meanwhile, English finished replacing British in the Lowlands.
There's a certain national pride that goes with Highland culture and Scots Gaelic. But Gaelic, except perhaps at court, was never the language of Edinburgh and the Lowlands.