we don't support"pure" Glacier in Mac/Linux. We only support S3 to Glacier lifecycle and having directly to Glacier storage class.
https://www.cloudberrylab.com/resources/blog/introducing-backup-for-mac-linux-2-8-1/

Thanks, that makes sense if S3 Glacier is becoming the preferred approach. One follow up if that is OK. The backup I am planning has about 1,000,000 individual files. Am I correct that the backup tool will copy each individually, resulting in 1,000,000 S3 PUT requests? That becomes a lot more expensive than the actual storage cost.

I don't know your specific details but in my case with approx 600gb backed up nighly by CBB Linux standalone to S3 with transition to Glacier immediately costs around $5 - 6/mth. My initial full backup cost was around $12 the first month. If you haven't done so yet check the Amazon S3 calculator. It's a good tool to Est what your costs will be.

Also I'm sure you know but once CBB does the initial backup it doesn't scan the whole S3 repository again and again racking up charges. It scans the local system for changes compared to its last backup from a local database.

https://calculator.s3.amazonaws.com/index.html