It's apparently obvious to you that "hey Gemini, can you message Mike that I love him?" means the text is first sent to Google and then back to your phone and then by your phone to Mike. This isn't the case for everyone, perhaps also because it's not necessarily that way:
https://www.macworld.com/article/678307/how-to-use-siri-offl... I couldn't find whether tasks related to "reading your messages" (like text to speech while you're driving or so) is a thing Siri does, but it obviously talks to you and if you tell it to send a message then that works offline so evidently there is some access there without needing to first upload it to the assistant's vendor