It's behind the S3 API & endpoints and integrates with AWS IAM. It's not a perfect drop-in fit for an existing application that talks to S3 but much closer than having to adapt it to a Lambda or API Gateway.
Lambda also has the limitation that it can't work with outputs greater than 6 MB and those outputs cannot be streamed - the entire content needs to be prepared before any of it can reach the caller.
I don't think of this as a replacement for direct to Lambda or API Gateway based solutions. Rather, when you need to be able to modify what an existing, possibly difficult to modify, application sees when it talks to S3.
Lambda also has the limitation that it can't work with outputs greater than 6 MB and those outputs cannot be streamed - the entire content needs to be prepared before any of it can reach the caller.
I don't think of this as a replacement for direct to Lambda or API Gateway based solutions. Rather, when you need to be able to modify what an existing, possibly difficult to modify, application sees when it talks to S3.