Thanks for sharing this! Such a simple and unexpected technique!
I had wondered about ISPs being able to see who is using messaging services like Signal and deduce who is talking to whom. "Oh, this person is sending some unknown message to this other person. And this other person tends to receive messages right after the first person sends the message. They must be talking to each other."
I imagine Tor would make this harder.
But, regardless, I imagine the reason other messaging services weren't tested was because of the researchers' resources and the scope off their research. Could this mean that Matrix could also be susceptible?