From the code, it doesn't looks to me it's really using S3 multi-threading for multipart upload:

1. A while loop in S3ClientWrapper#uploadLogFile to break the original segment file into multiple parts and upload them one by one through the custom S3OutputStream: https://github.com/aiven/tiered-storage-for-apache-kafka/blob/main/s3/src/main/java/io/aiven/kafka/tiered/storage/s3/S3ClientWrapper.java#L179
2. In S3OutputStream, the code will try to use S3's multipart upload API to upload the file in multiple chunks. But since the caller (S3ClientWrapper) already configured the file size to be config.s3StorageUploadPartSize, this would just end up with 1 upload in S3OutputStream

S3s multipart upload is supposed to use multiple threads to upload a big file concurrently onto S3. The current code path doesn't seem using S3's multipart upload threading.