Since they won’t say, I have a theory bouncing around in my head

Sonos supports both AirPlay and Apple Music, WiiM does not. Sonos is probably scared of WiiM and have some agreement in place with Apple that is getting in WiiM’s way

Purely conjecture on my part though
Please note that the WiiM Mini, WiiM Pro, WiiM Pro Plus and WiiM Amp all continue to support AirPlay, so any conspiracy revolving around that other brand might need further refinement.
For those of us hanging around since before the introduction of the WiiM Ultra this topic is really getting boring beyond compare. I'll try to list the facts one more time, to get it out of the way (at least for this thread

).
So, what are the facts?
You cannot simply make a device supporting AirPlay and sell it. You
must join
Apple's "MFi program" and obtain their certification. Apple recommend to do so
before you start mass production ...
WiiM mentioned that
the Ultra was pending certification, so they surely applied like they had done with their previous products.
The official statement on the
lack of AirPlay support can be found here. Note that they mention a "hardware limitation".
To put this into context, the MFi program covers not just AirPlay but a broad range of proprietary Apple technologies, including HomeKit, CarPlay and so on. They don't provide much information unless you have joint MFi and signed an NDA, but through
posts like this one on their Developer Forum it has become known that Apple support hardware or software based certification.
In the past, WiiM devices did not only work as AirPlay receivers but also as AirPlay transmitters. Obviously, WiiM felt entitled to provide this feature, but it was removed from all existing products around the same time the Ultra did not receive certification.
All WiiM streaming devices released after the Ultra are based on basically the same Linkplay streaming module.
What's speculation?
Well,
any conclusion drawn from the above is speculation.

The mentioned "hardware limitation"
could mean that Apple refused certification because the MFi chip is missing. I have seen no evidence if previous WiiM products had been certified based on the hardware or software solution. Some third parties surmised the new Linkplay module to be "faulty" or not follow Apples "quality standards". Now, that's
highly speculative for sure.
Apple might have been upset be WiiM implementing AirPlay sender functionality, Sonos might have intervened, all kinds of things could or could not have happened, we simply don't know. And due to the fact that all the details are certainly covered by NDA, we won't ever know.
As of now, new WiiM products simply don't support AirPlay and that's it.