Why would you think they were referring specifically to mammals? Mammals isn’t mentioned once whereas species/animals is used very broadly In the context of very broad ideas of evolution.
In either case, the hypothesis suggests animals should live longer in the wild than in captivity. So let’s take the only mammal that isn’t more likely to die from age/don’t die from old age, the naked mole rat. We don’t understand why they die and any given day there is a 1 in 10,000 chance they die at any age. In captivity they should have the same lifespan as lab rats (6 years); however, in the wild the have an average lifespan of 17 years and in captivity that expands to 30 years (again in contrast to the hypothesis where wild animals of the same species should live longer than those in captivity).
In either case, the hypothesis suggests animals should live longer in the wild than in captivity. So let’s take the only mammal that isn’t more likely to die from age/don’t die from old age, the naked mole rat. We don’t understand why they die and any given day there is a 1 in 10,000 chance they die at any age. In captivity they should have the same lifespan as lab rats (6 years); however, in the wild the have an average lifespan of 17 years and in captivity that expands to 30 years (again in contrast to the hypothesis where wild animals of the same species should live longer than those in captivity).