If you disable self-service Office 365 Groups creation in a tenant the "Create site" link on the SharePoint home page is hidden (even though the link is set to show under SP admin settings) By hiding this link, there is no way for a user to self-service a communication site, except navigating to _layouts/15/CreateGroup.aspx manually.
Disallowing Groups should not necessarily disallow creation of comm sites.
Seems Groups settings and SharePoint settings are mixed up a little bit imo.
