Guys, you may both be correct. Before transitioning almost exclusively to air guns, I shot a bunch of CF and RF rifles in numerous disciplines. I'll offer a couple of thoughts based on those 50+ years. First, at the highest level of BR competition, where the demand for precision accuracy is highest, I've never shot anything that did not perform best when clean (some needed 2 or 3 "fouling" shots to eliminate any solvent residue). That includes both firearm and air gun. That assumes a high quality barrel that has been sufficiently lapped, either by firing or manually. So, what's with the theory that barrels shoot better with a layer of lead? Three thoughts on that. First, I believe some barrels have small machining burrs, and the effects are alleviated by some leading. Next, (and especially with the original Smooth Twist barrels), they don't actually shoot better fouled, they simply don't get fouled. They have only a short section of rifling near the muzzle, and it collects very little lead. Lastly, a lot of air rifles are used in activities that do not require precision accuracy, or, the shooter is unable to perform at that level. In either instance (since we don't know what we don't know), the shooter doesn't know the barrel capability, and doesn't need any better performance.
Anyway, sorry to ramble, but those are my opinions. And my wife says I'm always wrong.