They don't know, they can't distinguish whos playing on the device, unless they use the front camera to spy on us...which I'm pretty sure is illegal, and I've been playing for 4 years on multiple devices and haven't been device banned yet, and I'm pretty sure STS knows because one of my YouTube videos even shows that I use multiple devices and I submitted the video for a contest.

You probably got device banned for something else, if you have 2 or more accounts, like I do, and one account gets banned then any device that account has played on can be banned if STS decides too.