No, it isn't.
IMO, The old time mountain music with claw hammer type banjos, slide guitars, and clog dancing is more authentic than the more stylized Blue Grass.
Between the Bill Monroe and the Blue Grass Boys, Flatt & Scruggs, and the Stanley Bother & the Cinch Mounts; the latter had more of an old time sound JMO.
There is also Newgrass pioneered by John Hartford, Progressive Bluegrass (the Dillards), Hillbilly Jazz (Vassar Clements) ....
Country Rock and Americana, don't have time..
There days it all tends to be called blue grass.