Hello, @ch1ptune. Welcome to the community. Sorry for the delay here, and welcome to the community. Apple devices, such as the Homepod will appear with a picture of a Homepod in the control menu. All other non Apple Airplay compatible speakers, such as your IKEA Shelf will appear with the generic speaker icon. All of these devices use UPnP(Universal Plug and Play) Which is a standard networking protocol, which allow them to communicate on the network.To Answer your question, this is a limitation of Airplay, as we have no say in how the iPhone perceives our products, compared to Apple products. Let us know if you have any further questions. 

Hi @Richard F.. Thanks for your response. So if I understand you correctly, only Apple devices show up as fully independent playback devices that can AirPlay to other devices?

So I can do:

iPhone/iPad → Sonos
iPhone/iPad → HomePod/Apple TV
HomePod/Apple TV → HomePod/Apple TV
HomePod/Apple TV → Sonos

But not:

Sonos → HomePod/Apple TV

Correct?

Hello again. I do not believe the Homepod can send audio to other devices. Other then that, you were correct with your statements. I have refined them to make them all correct, and have provided them below:

Supported Airplay Functions:

iPhone/iPad > HomePod/Apple TV/Sonos

Apple TV > HomePod/Apple TV/Sonos

Apple TV>Sonos/Homepod

Unsupported Airplay Functions:

Sonos > HomePod/AppleTV/iPhone/iPad

HomePod> Sonos/AppleTV/iPhone/iPad

Let me know if you have any questions. 

@Richard F. : Thanks. For the record, the HomePod is in fact able to AirPlay to other devices. 
 

See screenshot below. Here, my Living Room HomePod is AirPlaying to my Kitchen HomePod. It can also send audio to Sonos via AirPlay 2.

I stand corrected then. The HomePod can send audio to other devices. The only device that cannot send out Airplay signals to other devices in your case would be your Sonos devices.  Let me know if you have anymore questions.