I think "outdoor cat" in this context means a cat that lives inside with a human but is allowed outside, i.e. a cat that spends time outside but still has a safe place to call home. A feral cat is quite a bit different, and I'm not surprised that a previously feral cat would greatly value the safety and security of an all-indoor life.
We have 1 cat that is "internal" i.e. can enter the house (and is normally given food inside) but of course is free to go outside in the garden and 3 cats that are "outdoor" cats, meaning they won't enter the house but are always around the house and are fed as well (outside).
