Though it doesn't fit the specification of the linked project as it uses its own custom protocol based around Tor hidden services, rather than implementing XMPP, Twitter, Facebook messenger, etc. This may be more secure as it is keeping everything within Tor rather than using exit nodes, but perhaps less usable if everyone else you know is using more popular IM software.
Assuming that there are no security bugs in handling the incoming data, it's apparently as secure as communication with Tor hidden services. Each chat user self-hosts a hidden service, with messages between users being sent via a symmetric pair of circuits mutually identified using a random nonce.
Though it doesn't fit the specification of the linked project as it uses its own custom protocol based around Tor hidden services, rather than implementing XMPP, Twitter, Facebook messenger, etc. This may be more secure as it is keeping everything within Tor rather than using exit nodes, but perhaps less usable if everyone else you know is using more popular IM software.