I thought they were just joking or trolling. It could be a good tool to spy on your partner to see if they are cheating for sure. That's like the first "useful" thing came up in my mind.

If the metadata leakage was surfaced in the user interface, it wouldn't feel so sneaky. IOW, it could be like a game, "check it out, I publicly DM'd Jack and he DM'd me back". Obviously, there should be a fully private solution, but for 90% of use cases, the leakage is fine, and could be kind of fun.