It is a secure way to post messages. Anyone can see the posting (which is encrypted) but only someone with the password can actually read the plain text message. The password should be transmitted outside the scope of the site.
So it's only as secure as the method used to transmit the password to the other party?
That is known and outside the scope of the project. So, the idea would be to post a message that one or more people who know the predetermined password could read in a central location.
It's also only as secure as his javascript that you are downloading (from his server) to encrypt the message with.