Neither are muzzleloaders, but they list quite often.
A muzzleloader is a type of "firearm" specifically exempted/addressed/classified by the ATF. A projectile is expelled by the use of an explosive. "Air guns" do not use an explosion to expel the projectile (excluding some experimental "air" guns that ignite a gas). And the ATF/NFA definition of something does not always match its technical definition (and if you dig deep enough sometimes doesn't match within the ATF).
Muzzleloaders may need their own section; it would go under firearms (when reviewed in the past there were not enough to merit a separate section). Air guns may also need their own section, it would not go under firearms (again, historically there were not enough to merit a separate section).