The API: /api/sn_km_api/ is public api and doesn't require any authentication. Hence Required authentication will be grayed out as false
In order to protect knowledge bases and articles make sure that user criteria is added allowing only authenticated users (guest user or user with public role should not be allowed access). If the knowledge base / article is public any user can access (search/view) it via this api or even they can access from portal pages if portal pages are made public.
You can enable the below property and adding User criteria will help to "glide.knowman.block_access_with_no_user_criteria" for enforcing acl layer protection on kb