"encrypted_content" is just a poorly worded variable name that indicates the content of that "item" should be treated as an opaque foreign key. No actual encryption (in the cryptographic sense) is involved.
There is also another reason, to prevent some attacks related to injecting things in reasoning blocks. Anthropic has published some studies on this. By using encrypted content, openai and rely on it not being modified. Openai and anthropic have started to validate that you're not removing these messages between requests in certain modes like extended thinking for safety and performance reasons