Briefly: you can count them as "compatible" regardless that they are not
MariaDB and MySQL split their roads in version 5.5. Since MySQL 5.6 and MariaDB 10, the two systems started to have big differences. But we need to talk about what exactly is not compatible. There are big differences in the "physical layer" - file structures, storage engines, system databases, etc... so how the DBMS handles and stores the data.
There are practically no differences in the drivers yet. For example PHP uses the same connector to "talk" to both MySQL and MariaDB. Nothing is changed in that respect.
There are also not too much differences in the "user level" - the SQL syntax did not get any major updates in the new versions. Yes, there are plenty of new features (new keywords, new query types, etc) but the base remains the same just like it was in the old days. And they tend to "catch up" (when MySQL do something cool, MariaDB do it too and in reverse). Practically if you take a book for learning SQL for MySQL 5.0 and learn from it, maybe 99% of it will execute fine in the modern systems.
CMS-es and most commercial or open source software rarely use extravagant features of the DBMS - they tend to stick to the basics so they can be easily compatible. It's just much easier to write the code once and it is directly ready for many systems than to write different code for each different system.
So are they different - yes, they are a lot different. But the main differences are "under the hood" and they do not affect the software which interacts with the DBMS. Therefore Wordpress, Joomla, Drupal, etc - all big CMS will work fine on both MySQL 8 and MariaDB 10.4.
P.S. CMS written for MySQL 5.6 can be expected to work fine in both MySQL8 and MariaDB 10.x. More specifically - PHPBB and Wordpress will work fine with no problems on both systems.