I would assume so, the extreme case being just one key, which of course has only one partition. But see https://youtu.be/NXehLy7IiPM (2024 Reinvent S3 deep dive) - there’s still replication happening on single objects. So it’s still sort of sharded, but I do think key partitions where groups of keys have shared choke points based on sort order exist.
If you don't have "a lot" of keys, then you probably have only one prefix, maybe? Without them documenting the target order of magnitude of their shards?