However, there's probably a bit of a difference with the amount of on board processing that the two devices can do.

For example, my iPhone can do voice to text while on airplane mode.

{guessing} Similar on board processing would allow a HomePod Mini to work with HomeKit without needing to go to the cloud - Amazon working with Hue does need to go out to the cloud to get that voice to text to command layer. {/guessing}