People may be looking in the wrong places for records. All of the western part of Virignia was Augusta County until 1769 when the lower portion became Botetourt County, then Fincastle in 1772, and Montgomery in 1776. Giles wasn't formed from Montgomery until 1806. Knowing which county existed when is important when searching for early Virignia residents because records were kept in the original county. This family probably never moved anywhere - or at least not very far - after settling in what was originally Augusta County.
If Anthony Plymell was born about 1765 it would have been in Augusta County, so that's the place to look for his parents.