In phone A, you will have B's contact stored locally, let's say as "Dan".
B, A, "Pedro"
C, D, "Dan" (yes D is also named "Dan")
D, C, "Olga"
What do you look for?
Everyone can see and read the cipher text on all the papers, but each of the 4 people can only decode the things meant for them.
So, if that's how it works, you could certainly learn who was talking to who if you had access to all the devices. But access to one device only shows you what came and went to the device, but no data about which of the other three users were involved in those reads/writes. You would have to gain access to each device, in turn, to prove whether it was in contact with the first device.
I specifically said a lot of users. A lot is the opposite of one. Imagine a lot (>40% of total users) of impostor devices acting in accord to deanonymize some of the X and Y. Is it vulnerable to that. Like apparently Tor is.
You would at most be able to deanonymize a certain percentage within the impostor network itself. Kind of pointless.
Physical access of devices is the only way to have some chance of deanonymizing some of the users (always less than number of devices you have access to).
That’s my understanding, but the maker of this thing is here, and maybe can respond better?