That’s not that different than CoW filesystems - there is no rule that files must map 1:1 to objects; you can (transparently) divide a file into smaller chunks to enable more fine grained edits.
The unit of granularity for a CoW filesystem is a block, which is typically 4kB or smaller. The unit of granularity for S3 is the entire object or 5MB (minimum multipart upload size), whichever is smaller. The difference can be immense.
