Sorry, I can’t offer you ”proof” beyond my long experience with Alexa (which you admit you don’t use) and previous comments here where I’ve posted from Amazon developer blogs which explain that Alexa multi room is handled over wifi (using a prime sender and multiple receiver model) and that alexa multi room music is delivered at a level that all devices in the group can support, so if they all have the capability of playing 24/192, then that’s what they’ll play in an Alexa multi room group.
Amazon devices very definitely do not support or use airplay for multi room - WiiM may do that for their implementation of WiiM multi room, Amazon do not. So your statement about “no matter what app you use” isn’t correct, I’m afraid.
If you have two WiiM Minis, set them up in the alexa app in an echo multiroom group and play some AMHD UHD tracks to demonstrate what I say - I unfortunately only have one.
There are so many different echo devices. Can you point me to a specific Echo device that supports 192kHz for all members in the group? Once I have a link I will purchase it and try it. Thank you!