The statement "Bluegrass isn't music 'rooted in a tradition' but a relatively recent US invention (by Bill Monroe, principally) combining elements of a number of evolving traditions brought into a specific US region by a variety of immigrant musicians." seems self-contradictory to me.

As a separate, definable form, what we call bluegrass is indeed traceable to Monroe; given that, as you say, it combines elements of several traditions, how then is it ever not "rooted in tradition?" That strikes me a little like saying that a version of Barbry Allen that's backed by a Piedmont-style finger-picked guitar is not "rooted in tradition" because the twain hadn't met in real life. Of course bluegrass is "rooted in tradition."

And surely you didn't mean to imply that you wouldn't care to listen to it BECAUSE is isn't "rooted in tradition" did you?