SQL Server AG Worker Threads
So I believe what you read is dated information specifically pertaining to the first version of Availability Groups when it was released with SQL Server 2012. I found what I'm guessing is the RedGate Article written in 2014 that mentions that quote &quot;Each Secondary replica uses 1 redo thread for each secondary database&quot;. This was known as the serial redo model.
The source article from Microsoft which that RedGate article references has since been updated (in 2020) and no longer states that's how the secondary replica works.
Furthermore I found the following Microsoft article which mentions how they've switched to multiple parallel redo worker threads per database by default, aka parallel redo model, starting with SQL Server 2014. This must be what you're experiencing.

When availability group was initially released with SQL Server 2012,
the transaction log redo was handled by a single redo thread for each
database in an AG secondary replica. This redo model is also called as
serial redo . In SQL Server 2016, the redo model was enhanced with
multiple parallel redo worker threads per database to share the redo
workload.

This is a feature that is probably not often tweaked, so I would personally recommend leaving it as is until you run into a problem you're sure is related to the parallel redo model. If you do run into an issue, then it is possible to switch back to the original serial redo model as the above article states:

To switch to serial redo model, TF 3459 needs to be enabled. But after
the SQL Server instance is running in serial redo model, the only way
to change it back to parallel redo is to restart the SQL Server
service.

Nice question by the way. :)