Earlier quoted context omitted.
These days, most smaller users go with a managed DBaaS such as AWS RDS, and on RDS MySQL gets massively more usage than MariaDB. I build schema management software for MySQL and MariaDB, https://www.skeema.io , used by several hundred companies. Among my userbase, MariaDB is a tiny minority, to such an extent that I wonder if it's worthwhile to continue keeping up with their new quarterly release schedule.
Is it possible MariaDB registers itself as MySQL? No clue how your system works or how you figure out which DB it is.
The Skeema CLI is able to accurately identify your database version and flavor (which is necessary for its functionality), there is no notion of MariaDB registering itself as MySQL. But the CLI does not contain telemetry, so I don't have hard stats on MySQL vs MariaDB usage.