You are absolutely right! Using a single public encryption key can not be considered as secured. But it is still more than having your content in clear.
I intend to add more encryption options (sharable custom key, PGP), that way users can choose the level of encryption they want for their public content. Of course, the next versions will still be able to decrypt legacy encrypted content.
In a way, it makes online Privacy less binary:
Instead of having an Internet where we choose to have our content either "public" (in clear) or "private" (E2E encrypted), we have an Internet full of content encrypted with heterogeneous methods of encryption (single key, custom key, key pairs). It would be impossible to scale data collection at this rate!
I don't think AI is bad as a whole. At least I would like to choose if the content I post online can be used (or not) to train models.