I believe the "seasoning" thing came from old BP shooting sources where they were using extant rifles with barrels very much like iron, not modern steel, and this was a carry-over in the 1950's an 1960's, where the guys who were part of the "resurgence" of muzzleloading were repeating what the guys in their grandfather's generation had been taught. I think it's possible that you might get some carbon depositing in a bore near the breech due to temp at ignition, but that wouldn't carry to the muzzle, or anywhere close. Depending on how you clean, you may be removing all of such a deposit.
LD
LD