What is generally termed folk music (not sure about capitalisation or its implication) may indeed be different from other MODERN forms, but is much closer to classical music (not in the pedantic sense) which was hardly a seperate branch of music then, and marches (which it includes) FROM ITS OWN PERIOD (say 1600-1750). Music, like most other human activities, has changes of fashion quite frequently, and those familiar with them can date musical styes to within a few decades or better. Listen to concertos by Handel, Vivaldi, and Boyce. Obviously by different composers, but very similar, because they had the same external influences. This "fashionable" sound of the period is what we recognise as the "folk" sound. And we can probably date it, perhaps a little roughly, the earlier tunes probably more modal, because the main influence was church music, and later tunes more melodic with the popularisation of "classical" concerts, and more formal dances (think Jane Austen)
All music, not just "folk", will be played in a style that owes something to the fashion of the times, because that is our main influence.
That is where the Good Purist comes in.
S/He researches the period and performing styles when the music was composed, and is able to demonstrate how it was originally played (maybe). This is a Good Thing. We know our music's origins, however we subsequently choose to play it.
Then there's the Bad Purist who attempts to prevent the music evolving by only approving performance in the original style.
Otherwise referred to as "precious"; this is a Bad Thing.

BTW does the 1954 text include a definition of "working class"?