... presumably because (if you have one instance of the Briar installed) when you're talking to two different people they can check and confirm you're the same person, while in SimpleX you can create disposable/ephemeral identities for different chats.
I haven't reviewed this thoroughly but I can see that there are a lot of attributes that could be added to this table in regards to metadata protection against various parties, including revealing online presence to servers and contacts (which is a place where briar falls short).