Exactly - someone could be running a (the?) third-party Signal client, or they could be intercepting the Google push notifications that Signal sends using one of those Play Services compatibility layers, or they could do one of the low-tech things of showing their device with your messages on it to someone else in-person.

Although, to be fair, all of those things are far more difficult than using Element One, so there's an argument to be made about reasonable expectations and the actual likelihood of your messages being private...