Well!
& I always thought of "Delta Blues Music" as a style.

Methinks this comes under the same banner as "What is Folk Music?".

As the original editor of the Oxford English Dictionary, James Murray (1837-1915), said of language (apt here) it has a firm centre, with certainty stretching a distance therefrom but a periphery increasingly vague & ill defined. Or he might have opined "a circumference defined by the listener".

FWIW IMNSHO Leadbelly was Folk-Delta Blues, Muddy Waters played Blues, and not "Folk".

As for perpetrating a myth. History is made by the winners, and the Lomax's did a lot of winning. When few others were.

Labels are one way language communicates. It gives you something to argue about at least.