After opening a ticket with Cisco Umbrella. This is the answer I received.
"Hello Kenny,
Currently it's not possible to retrieve Cisco managed S3 log files with Splunk. This is due to the restrictive permissions on the bucket itself. The Splunk AWS module expects to be able to list all buckets, which it cannot - as the Cisco Managed buckets are restricted to that singular bucket. However, you can access the logs with the Amazon cli tool. And other third party tools like S3 Browser."
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