First, The book is out of print, but probably is available in most genealogy libraries.
The book lists and proves (by reference) the children of Erasmus Withers Allen which include both Thomas Allen and Elizabeth Allen.It is also proven that Elizabeth "Betty" Allen married James Chapman, a neighbor in both VA and NC, but there is no proof of Thomas' marriage.After his death as a Revolutionary Soldier, she married again in Rowan County, NC and moved to the Cheraw area of South Carolina with her second husband...Nathan Shepherd.
I suspect that this is the Thomas Allen who married Margere Bryan 5/26/1789 in Rowan County, but have only the knowledge that he lived next to the Bryans/Boones and moved to KY with them.It was his second marriage.We probably need other elements of proof such a a correlation of children's names, etc.Certainly finding a grandchild named Erasmus or Withers, Streshley, etc. would solidify the proof.
Of course, you need to check all the original records that are referenced, in order to verify what is in the book.At over 500 pages, there are certainly bound to be some errors.