After LPS-46860 (ee-6.2.x@1af...e1d), organizationUser will be true for an organization site even if the user is not a member of the organization.
Steps to reproduce:
It says "You are a member of org because you belong to org."
Viewing Site Memberships is slow with large number of organizations
SitesImpl.isOrganizationUser returns true for an organization site even if the user is not an organization member