All sorts of definition problems here, since bluegrass is generally defined as beginning with Bill Munroe.
Students of Bluegrass, such as Richie- thread 55312, Bluegrass Music: Bluegrass take a broader view of the music's origins.
It is true that many of the Appalachian settlers were originally lowland Scots, but many English settlers followed the same route, as did some German and other immigrants.
The Scotch element already had lost much of their Scots language by the time they came to North America, and it had essentially disappeared by the time they settled in the Appalachians. As Nerd says, their cultural heritage had become mainly Anglo-Saxon.

In light of this background, Richie is correct when he says southern Appalachian music draws heavily on English ballads and songs (of course including Burns writings and a few more ballads, perhaps originally Scottish but spread to England and in Anglo-Saxon style) and fiddle music from Europe and the United States, with infusions from the blackface minstrels (who included Irish routines as well), gospel, and of course composed pieces diseminated by broadsides and traveling entertainers. The importance of the English roots is a point taken in the post by Greg Stephens.

In brief, "Celtic" roots are much diluted and difficult to isolate.