AWS s3 cross account policy allowing public access

Question

I'm having some difficulty locking down an S3 bucket that I must share across accounts.

I have no public ACL defined. "Everyone" has no options enabled. The console bucket list says "Bucket and Objects not public)

This is my policy (created using the policy generator): (I have changed the bucket name and principal ID)

{
    "Version": "2012-10-17",
    "Id": "Policy1554292781812",
    "Statement": [
        {
            "Sid": "Stmt1554292513566",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::0000000000000:root"
            },
            "Action": [
                "s3:GetObject",
                "s3:GetObjectVersion"
            ],
            "Resource": "arn:aws:s3:::my-bucket/*"
        },
        {
            "Sid": "Stmt1554292778889",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::0000000000000:root"
            },
            "Action": [
                "s3:ListBucket",
                "s3:ListBucketVersions"
            ],
            "Resource": "arn:aws:s3:::my-bucket"
        }
    ]
}

My expectation is that one other account (0000000000) has read access to list content and get objects from this bucket. No one else should have any access.

The 0000000000 account developers have started work on some code today to access files in that bucket. What is interesting is that they have not yet configured their own access credentials. So the client (IAmazonS3 C# client) is attempting to access the bucket without any authentication. They are able to list the contents of the bucket.

I can't see any file level permissions set on any files in those buckets.

If I try to access the bucket Cyber Duck for ease and using the account key for a separate test account, then I am met with an authentication error and denied access to the bucket - exactly what I expect.

Why can these guys access the bucket contents using a C# client without supplying their own access credentials? What have I missed?