Stefan Fercot does some explaining:
Recently, we’ve received many questions about how to take backups from a standby server using pgBackRest. In this post, I’d like to clarify one of the most frequently asked questions and address a common misconception for new users.
First of all, it’s important to understand that taking a backup exclusively from the standby server is not currently possible. When you trigger a backup from the standby, pgBackRest creates a standby backup that is identical to a backup performed on the primary. It does this by starting/stopping the backup on the primary, copying only files that are replicated from the standby, then copying the remaining few files from the primary.
Read on to learn more and to see an example of how this works.