Bluegrass is not an old-time style at all

[Bluegrass] is not an old-time style at all; it did not begin to take shape as a distinct entity until the mid-1940s, and it was not named till a decade later… the music, of course, drew upon earlier string band and vocal styles and repertory (as most forms of country music did), but they inherited and borrowed styles were significantly modified [i.e., updated]

B. Malone, Country Music USA (5th Edition, 2018), 380