Regarding the question of Scotch-Irish migration being lumped together in American history:

Yes, generally, historians tend to group the whole movement as one long event rather than break it down to smaller events. Fischer's Albion's Seed takes the long general approach and, while this is considered an important book, the critics say that the generalization is it's major flaw.

He describes the movement of Scotch-Irish into the frontier lands of the colonies. You know America better than I do at this time in the morning without getting the book out, sort of the Ozark, Smoky Mountain areas. He doesn't really get into the late-1700 to mid-1800 migrations. That would disrupt his thesis.

It is a good book if you pay attention to what he is actually describing.

So, yes, that study could create confusion.