Well, they could have. Unless you're generating original content. This is the problem if you are sharing nothing something which is not 100% original on block level.

This is just the reason, why sensitive data needs to be encrypted before deduplication.

OFF System is also one interesting approach to this problem: https://en.wikipedia.org/wiki/OFFSystem

These are more or less "funny" work-a-rounds to the actual problem.

Which in theory they don't have, as they encrypt it on your browser.

Well, you said "encrypt", what do you mean by encrypt? What's the key? In the Freenet's solution, they encryption key for the data is the data, not any random or user provided key. So the same plaintext is always turned in to same ciphertext. Which we all know, it's very bad idea, it ruins encryption. In this case, it's quite easy to spot out all of the users having the same content in their account.

This is only reasonable method when ever you can assume that every plaintext is unique, which also makes point of deduplicationg data absolutely pointess.