Are the findings of the Rat Park experiments applicable to porn addiction?
As a therapist working with patients suffering from all kinds of addictions – drugs, porn, computer games, social media, gambling, sex, etc. – the simple answer is that addictive behavior is influenced by:


currenct life circumstances (Rat Park versus cages, if you want)
personality (some people are more prone to addiction than others)
life experiences (childhood abuse and other trauma)


So yes, a person in a happy romantic relationship with a supportive network of friends and family working a satisfying job will most likely not become addicted to porn from masturbating to it every now and then, just as they won't become addicted to THC from smoking a joint on Saturday evenings, but the influence of these life circumstances do not protect someone from becoming addicted when they make traumatic experiences such as the death of a child or when their personality is "addiction prone" (e.g. high neuroticism, low conscientiousness, high impulsivity, high reward and threat sensitivities, and so on).