Not an open transaction (and detaching and attaching would never solve that, detaching and attaching a database won't change anything within the database)

Either the database has had replication in the past (possibly snapshot replication with the bug mentioned in the article), or has been restored from a database that was replicated. SQL still thinks that everything in the log needs to be replicated but since there's no log reader that will never happen, hence the log is never truncated and will grow until it fills the disk

If you are absolutely sure that there's no replication, then you can do the following:

Create a transactional replication publication

Publish a single article

Stop the log reader

run sp_repldone

Drop the transactional replication publication that you created

Now run DBCC OPENTRAN again and there should be no reference to distributed and non-distributed LSNs