Otis, I keep hoping to see you become a truly serious RESEARCHER, not just another gullible flash-in-the-pan wannabee like so many who amass databases in the hundreds of thousands with a high percentage of obvious errors due to what I call copy-cat-ism. Not that I've noticed you doing anything of that sort yet.
And every time I think you've made great progress in this admittedly painful task, you come up with something that shows you are still not thinking (and intrepreting what you find) nearly as precisely as you should be to succeed as thoroughly as you clearly wish.
The quotation you cited stated quite precisely "moved into Macon County": it did NOT say from _whence_ they came, just where they arrived!! Didn't even mention the route they took, which might have yielded additional clues.
In order to FIRMLY tie your Macon County Alabama Abernathy Methodists to the Abernathys of Giles County TN, you really need to come up with a citation (better, two!) that is more along the line of"5 or 6 families moved together down into central Alabama from central TN in the years immediately following [event] and included [list of surnames], who settled in Macon County before 1850"... Your chance of finding something along those lines is, I fear, rather slim, but not absolutely and irrevocably impossible.
However, you are beginning to gather quite a bit of circumstantial evidence, and throughout history people (both individually and en mass) have been both villified and exonerated on the basis of circumstantial evidence.
Sometimes by the exact same evidence, which I find ironic.