This is nonsense - most people don't know the difference between imessages and SMS, it absolutely is surprising behaviour for the user that the messages now go to an app on her laptop that she may not even use.
Plus this is the sender writing a message to her email address. Her laptop didn’t steal her phone number.
Everybody knows when one person in a text group isn’t using iMessage, and they usually hate it.
You're saying it's not? And you're saying everyone knows this? I say incorrect.
This is indeed totally surprising and inexplicable behavior.
Answer: "idk, tech magic." They know there is some way that isn't actually magic, they know there are mechanics, but they don't know or need to know what the mechanics are.
How it knows to display a green bubble or a blue bubble is exactly like that. That's what I mean by it's not a remarkable thing to anyone that the txt app does this. Q: How? A: Somehow. They don't see it and think "Wow, this proves that there are two unrelated back-end systems."