This misconception is largely why I find Appletun and Ferro to be capable of running in tandem.
Ferro, as you've stated, has no reliable recovery. Access to Recover is what changes the dynamic of Appletun from Ferro. Ferro can wall certain things, but Ferro itself isn't meant to be played like a wall. If Ferro was your main response to, say, Hydreigon, Dragapult, Inteleon, etc, then Ferro would end up not lasting long in the average game at all, due to its lack of recovery aside from Leech Seed/maybe Protect and Leftovers.
Appletun has good bulk, and good synergy between its typing and ability, along with reliable recovery. If Appletun is your main responses to physical/special attackers, it will generally not disappoint aside from typing disadvantages, such as the aforementioned Darmanitan.
If allowed in lower tiers, Appletun will likely make for the best mixed wall in whatever tier it ends up being allowed in. In OU, Appletun is a wall that isn't passive. Ferrothorn is an excellent defensive utility Pokemon who isn't passive. Wall /=/ defensive utility.