The analysis by Jennings states:
when the death ages of these porn stars were averaged it was discovered that the average life expectancy of a porn star is only 37.43 years whereas the average life expectancy of an American is 78.1 years.
He has simply calculated the average age at death of those unfortunate 129 persons and presented it as "average life expectancy of people working in the porn industry."
This is an error in statistical sampling. Life expectancy is not the average age at death of a subset of a group that has died. According to the Wikipedia article on the subject,
life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of their birth, their current age and other demographic factors including gender.
Incorrectly calling the average age at death (37 years) the "life expectancy" gives a very misleading and pessimistic picture.
Moreover, the article ignored the lifespan of the large number of persons who might have worked in the porn industry for many or a few years before retiring or moving on to other occupations.
In short, the figures probably show selection bias, as also pointed out in some early comments on the question.