There's the "Modern UI" and the "Modern Site". While Microsoft does not stress the difference, the key difference is how users and security are handled. In the article you linked to they also said "features like security inheritance through sub-sites doesn’t exist in the modern site framework". Note that the "Modern Communications Site" does not use "Modern Site" features, just "Modern UI" pages.
If you create a "Modern Team Site", or any of the other "Modern Security" sites like those that are auto-created behind Outlook Groups, Teams, Planner, etc., you will find that you get these three things:
- An Azure AD Office 365 Group.
- An Exchange Email for the group.
- A SharePoint Site Collection. (In Teams, Planner, etc this is behind the "Files" link.)
When in these backing SharePoint sites, you manage permissions to the top level site of these Site Collections by adding users to the "Owners" and "Members" Office 365 groups. When you create a subsite, you revert to managing access by using "classic" SharePoint permissions: 32 permissions rolled up into Permission Levels and managed by granting these Permission Levels to users or SharePoint Groups.
So, these subsites do not participate in "Modern Security", they do not get their own email address or their own Azure AD Office 365 Group and do not participate in other "Modern Site" features like hubs and discoverability. They are just Classic SharePoint sites with some Modern UI pages. It would not surprise me if the "Modern Sites" soon lost their options to create subsites.