The prefixes are not meaningless, they are performance boundaries as well if you read the docs.
But that's a different thing than what the post is about. Even teams that use prefixes for performance don't have an S3-native way to ask what a prefix represents, who owns it, whether it's still accessed, and so on. The semantic layer is missing whether you're hashing for throughput or just laying data out the obvious way.
[1] https://docs.aws.amazon.com/AmazonS3/latest/userguide/optimi...