There are a variety of reasons why the Hammet cult assumed they were of a different “species.” One of the most prevalent was the notion instilled by Edgar Hammet that they were able to engage in near-cannibalism to strengthen themselves.
To begin with the group indulged in autophagia, otherwise known as “autophagy.” They would bite their own flesh, attempting to mimic the vampire’s they had read about and seen on screen. However, this only served as a learning curve for them as it proved ineffective.
Autophagy is more of a practice that serves bodily nutrition by consuming some of the body’s tissues, it’s used to remove damaged organelles within a cell. This is not the same as what vampires practice, obtaining their nutrition from blood. Various cult members experimented with varying levels of autophagia, none very successfully.
Edgar Hammet fully tested his theory out on some of his victims, but both Neyla Reed and Kyp Malkmus have been quoted as saying that he didn’t benefit much from his hypothesis.