Video in comments.
I was able to trace down to "s3:ListBucket" not being checked by testS3Connection utility. meaning without "s3:ListBucket" permission testS3Connection can say everything is ok but a problem still exists when starting up columnstore via the debug.log you see
When using testS3Connection and it says OK. that the cluster can startup fine and store data in S3.
Not having s3:ListBucket, you cant use S3 storage for columnstore
Minimum S3 permission needed for access key/secret S3 auth determined: