For the record, after selecting “All” in Preferences - CardDAV - Addressbook, the Accounts do appear between CardDAV contacts. Excellent, beyond expectations.
The only glitch is, that there are all accounts exported that EGroupware has access to. And because it is possible to specify only one LDAP/AD context, and thus the context is quite broad, there are all accounts, even technical, admin etc.
We are trying to develop an option, where for example only accounts with telephone/mobile number would be exported by CardDAV. We’re looking into source code, whether we would be able to create a patch.