You could be right about it not making sense. I understand the 30-day minimum would still apply. If I understand this right, it’s more than how long the files have been stored at play here.
I think it would ultimately depend on how frequently you prune, what files change during that process, etc. If the auto-tiering is truly intelligent, files shouldn’t automatically be moved to IA because it would be frequently changing and thus not a good candidate for tiering down.
The question is whether or not enough files would remain untouched long enough to make paying the Intelligent-Tiering worth while (i.e.,would the savings for the files that qualify outweigh the per object monthly monitoring and automation fee).
I imagine the only way to really know is to run two S3 repos in parallel for a couple of months, keeping everything else equal (same files, same backup frequency, same prune schedule, etc.) and compare charges.