Mariabackup inherited a xtrabackup_binlog_pos_innodb from Percona XtraBackup. However, this file is apparently only safe to use with Percona Server versions that have a special lockless binary log feature:
Starting with Percona Server for MySQL 5.6.26-74.0 LOCK TABLES FOR BACKUP flushes the current binary log coordinates to InnoDB. Thus, under active LOCK TABLES FOR BACKUP, the binary log coordinates in InnoDB are consistent with its redo log and any non-transactional updates (as the latter are blocked by LOCK TABLES FOR BACKUP). It is planned that this change will enable Percona XtraBackup to avoid issuing the more invasive LOCK BINLOG FOR BACKUP command under some circumstances.
When the Lockless binary log information feature is available  on the server, Percona XtraBackup can trust binary log information stored in the InnoDB system header and avoid executing LOCK BINLOG FOR BACKUP (and thus, blocking commits for the duration of finalizing the REDO log copy) under a number of circumstances:
- when the server is not a GTID-enabled Galera cluster node
- when the replication I/O thread information should not be stored as a part of the backup (i.e. when the xtrabackup --slave-info option is not specified)
If all of the above conditions hold, Percona XtraBackup does not execute the SHOW MASTER STATUS as a part of the backup procedure, does not create the xtrabackup_binlog_info file on backup. Instead, that information is retrieved and the file is created after preparing the backup, along with creating xtrabackup_binlog_pos_innodb, which in this case contains exactly the same information as in xtrabackup_binlog_info and is thus redundant.
Maybe Mariabackup shouldn't create it at all? It can lead users to make dangerous assumptions. For example, some users seem to think they can use this position when they run Mariabackup with --no-lock.
For example, if we take a backup with --no-lock:
And then prepare it:
Then this file is present: