Northumbrian pipes haven't been around forever. In their present form they only date from the end of the 18th century. Mixolydian chanters are older, and must have been played in Northumbria before the advent of the Northumbrian pipes. That tune looks to me like an *early* 18th century one, the sort of thing you find in the earliest Scottish manuscripts.

Do the people who are praising "just intonation" really know what it is? It isn't used in any folk tradition I can think of. Experiments have shown that when people with flexible-pitch instruments like fiddles are left to their own instincts, they tend to use Pythagorean intonation (pure fourths and fifths), which is quite different. And if you are trying to get the most out of an instrument like the hammered dulcimer by emphasizing pure thirds, you'll probably use meantone, which is neither just nor Pythagorean. And the Highland bagpipe is wildly different again, with its closest analogue being a scale developed by the Arab theorist Zalzal in the early Middle Ages.

Not all alternatives to equal temperament are the same, but a mode like Dorian or mixolydian/major/lydian pentatonic is recognizable whichever of these intonation systems you play it in.