WiFi acts like a "hub" in this case. Basically WiFi is a totally brain dead standard in which all adapters shout at each other and if the other side couldn't hear them they retransmit. I doubt most people bother with anything other than WiFi. A lot of my relatives aren't technical, and they don't. For my parents and my wife's parents I've done everything the "right" way, but few people have access to me.
Edit: actually, never mind. Apparently only WEP encrypts everything with the same key. WPA uses a key + a different nonce per client, which in effect means clients can't see each other's traffic, at least not in plain text. You can still maliciously decrypt traffic, of course, but it's hard to do, and I strongly doubt that's something Roku would do. It's probably also illegal.
More on decryption:
https://wiki.wireshark.org/HowToDecrypt802.11. TL;DR: if you didn't capture the handshake when the adapter pairs to AP, you're SOL. If you did capture the handshake packets -- decryption is piece of cake.