Adult back links referring domain ratio?

Hey I have been using SEO on mainstream websites for years with success. I always try to follow best practices. I recently created an adult website 4 months ago and have been slowly but steadily crawling up the SERPS. I have been analyzing my competitors backlinks using AHREFS and have found a big SEO no no that they all have in common. They all have a very low referring domain ratio in contrast to the number of actual backlinks. For example the highest ranked competitor I am looking at has 238k back links from less than 200 referring domains. This website is ranked on page 1 for all value key phrases and gets over 80 thousand visitors a day. Why has this website not been penalized by google? It isn;t that one site either it is almost all of my competitors in the adult niche. I have never seen such a common trend while analyzing competitors backlinks until i started to venture into adult.

So my question is....... Is adult different? Can you have such a lop sided ratio of referring domains to backlinks? If so, I can pull the trigger on a service that will do the same thing for me, just weary to use it because not sure yet about this whole thing.