I read "open it for testing (or backups in this case)" as letting recovery complete and promoting the cluster to a master before taking the backup.
Don, is that the case? If it is, I think there's a problem with or without a timeline switch. If you confirm the backup is being taken as above then I'll detail my concerns.
Note that this is just for creating a couple of one-off backups to restore for our dev and pre-prod environments. Given that, I was going to open a new clone as its own cluster and take backups from that. The data would be the same though and suit purposes of the dev and pre-prod refreshes.
If I were taking backups for the purpose of production backups, I would not do things this way. That is the eventual plan but right now we aren't ready to make the changes necessary in the production environment.