But only the following Apple devices can be a HomeKit hub:It's because the Homepod, iPhone or other Apple device is doing the secure processing of the request that the third party device is incapable of doing by virtue of its hardware. The idea is that an Apple device may be in your home however in another room and out of vocal range, but say a Sonos is much closer — so the Sonos speaker would theoretically accept the Siri request, sends it to the other apple device in the other room which securely processes it, then sends the results back to the Sonos seamlessly. This way allows the use of Siri without letting a third party into the secure stack.
Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support
Set up a home hub to control your smart home accessories while you're away from home, share control of your accessories with others, and automate your accessories to do what you want, when you want.
Note that Macs and iPhones are not in the list