We have considered that assertion before, Kendall. The great "North British"* writers {Burns, Scott, Stevenson} happily used 'Scotch' of people. I have always suspected that its use also to designate an alcoholic drink caused it to fall out of favour as a personal descriptive as part of Victorian genteel temperance, & the distinction you adumbrate is probably no older than that.

*North Britain for Scotland another Victorian locution, for some reason ~ probably not unconnected!
~Michael~>