for the packages or documentation no issue is needed as sqlite3 is still the default database used for ownCloud. Furthermore there is no note in your quoted documentation that mysql / mariadb will be installed by default.
Additionally ownCloud or the packages can't decide for an admin if he wants to use SQLite, MariaDB/MySQL or PostgreSQL. To solve this the packages would need to install all three databases which can't be done.
Finally the packages have a recommendation to mysql-server, mariadb-server or postgresql which shows you alternative packages which you could but don't need to install. An admin should be able to handle such recommendations from my PoV. Furthermore recommended packages are normally installed by default in a standard environment.
For the other topic please create a new thread as mixing different issues in one thread is highly discouraged for the reasons explained here: