Unfortunately the wiim mini doesn't support the google cast ecosystem, only the more expensive devices. But the more expensive devices don't support airplay, which seems like an odd choice.
i'm not sure what device you're looking at, but i've got a wiim pro plus and it definitely supports airplay - i was just using it yesterday.
It seems like the newest and most expensive devices (WiiM Amp Pro & WiiM Ultra) do not support Airplay.
i had been considering an ultra to replace my tv soundbar, but no airplay is probably a dealbreaker for me.