Bluetooth headphones are generally receiver only. The wifi headphones i have seen available have a base station wifi transmitter and the the headphones have the WiFi receiver. For a WIIM headphone it would have an embedded WIIM board in it which is a transmitter and receiver. If you have a multiroom the WIIMs, headphones and wired, would have to chat away in the background constantly while the music is being transmitted. This is in addition to the Internet streaming direct to the headphones from the streaming services and all the other system to Internet traffic. That's quite a bit more traffic than Bluetooth and may indeed be worth a small tin foil hat
Qualcomm are messing around in this area. From what little i understand the qualcomm based headphones connect to the wifi network and the source is on the same wifi network. This gives it the range and allows high res files to be streamed and the device size down.
I think PSB were also looking at high res from the internet a while back using the MQA backed bluetooth codec with compression but that was high bandwidth bluetooth to the headphones and i think there was a whole new OS involved. Not sure if you had to have a Basestation on the same wifi network as well, was not very interested, it lost me at MQA. This was some time ago as well, so not sure where they got to.
Long version of my initial thought and could obviously be out of date.