https://aws.amazon.com/blogs/aws/amazon-s3-update-strong-rea... guarantees that a client's lockfile can always be seen by other clients immediately (which didn't used to be true). If every client backs off and retries after a race, is that enough?
For locking to work properly you'd need to have a conditional write that would fail if some prerequisite was not met. GCP offers that operation, S3 AFAIK does not.
client A lists s3://bucket/prefix/.pulumi/locks/, sees nothing
client B lists s3://bucket/prefix/.pulumi/locks/, sees nothing
client A creates s3://bucket/prefix/.pulumi/locks/unique1.json
client A lists s3://bucket/prefix/.pulumi/locks/, only sees unique1.json, and proceeds
client B creates s3://bucket/prefix/.pulumi/locks/unique2.json
client B lists s3://bucket/prefix/.pulumi/locks/ and sees both unique1.json and unique2.json
client B assumes it lost a race, deletes s3://bucket/prefix/.pulumi/locks/unique2.json, and retries