They can write the data, but no other client can read it.
In rpcwallet.cpp I saw
senddata(const Array& params, bool fHelp)
...
Yes, this does baffle me. I thought data was stored without encryption, and that any other client could read it. We will have to redo the steps that the Apertus.io team went through to understand what went on. This seems to imply that the block explorer or DTCbrowser (or Apertus.io) must do some sort of decryption. I also wonder of they were using a client that could not do RPC for getdata. If I remember correctly the earliest datacoin-hp client could not do RPC. It was mostly for mining.
Data is stored *unencrypted* inside the blockchain, unless you encrypt it by your own means. Data can be posted RAW or using the envelope protocol (defined by OP).
I wrote a
tool some time ago to read both types of data from a running datacoind using RPC calls .