There can be some differences between pembrokes who are primarily bred for agility and those who focus primarily on AKC conformation, but given your dog's origin, more than likely, he is just from a BYB. This doesn't mean your dog is 'bad' by any stretch, but it is not very common within the agility corgi circles for a dog to be rehomed in that way (especially from 'rural north carolina').
"Farm Corgis" is usually code for "Backyard Breeders" who breed corgis because there are a lot of people who want the breed for their appearance, but do not intentionally or carefully breed dogs for any purpose.
Corgis still compete in herding as a sport, but there is not a large community of honest working herding corgis. You should still see titles on parent dogs in the breed for whatever purpose the breeder toutes them for. (Agility dogs should have parents with high ranking agility titles, for example). There isn't the massive divide in pembrokes like there is in BCs or GSDs, especially when you are talking about hereditary lines.
Having a young, lanky corgi is also not the same as a fat, older corgi in comparison of appearance. A corgi from the conformation ring will look pretty notably different than a BYB corgi. It will be rare for a sport focused breeder to sell a dog not on a 'return to breeder' contract or without proper buyer vetting. Your corgi probably seems 'leggy' due to lack of chest, not because he is actually taller (which is normal for his age), and increased muscle tone is due to plenty of exercise and good care - a well cared for Pembroke should not be fat or tube shaped.
So, while it could be an explanation, given your corgis back story he is most likely just a BYB corgi from a breeder in rural NC. Which is totally fine and just is what it is.
One of my favorite agility corgis, Ddare
Corgi not bred to standard / Likely from a not great breeder
