Then I don't understand your complaint. The article author is very explicitly using definition 1. It is explicitly accusing the community built by Yudkowsky of presenting signs of being "a group (such as an organization or religious sect) with tenets and practices regarded as coercive, insular, or dangerous".

The article is certainly not using definitions 3 or 4, which simply don't make sense in the construction "sex cult". It's not worship of sex/worship through sex (definition 4), and it's not "a system of religious practices related to sex" (definition 3).