On Steemit, I believe, the content is not recorded on the blockchain

It is.

Your reply above was recoded in transaction 3e3162c77d23cc2e764a67e5070df08c1d2d8178 which can be viewed an the steemd block explorer: https://steemd.com/tx/3e3162c77d23cc2e764a67e5070df08c1d2d8178

Only the text is (supposed to be) recorded on the blockchain. Pictures, video and audio is typically saved as links. IPFS has become a popular way to distribute such media. However, as the source article says, being in possession of links to child porn can also be a major offense in some jurisdictions. I also believe that even a written story, fiction or factual story, may be considered "child porn" in some jurisdictions. It's also possible to embed graphics as text, i.e. as base64-encoding.