I’m not sure I fully understand your question, but VoWiFi is a type of VoIP. VoIP is the general concept of conducting voice calls over an IP packet data network, and VoWiFI is an example of a specific VoIP standard. Other examples of VoIP standards or platforms include SIP, Matrix, Discord or Skype. Each requires a different implementation. Some are based around service providers that insist on non-free software. VoWiFi is the standard used by cellular carriers for seamless VoIP using your cell phone number. Its integration with the cellular network makes it more complicated to implement.
That was my suspicion. You seem knowledgeable enough to confirm it. It’s interesting that VoLTE may be easier to support than VoWiFI. Good post, thanks!