Well, I think a key part of it is what exactly you mean by:
> I recently had ChatGPT Astra write a bunch of short stories for me in a universe I'm fond of.
Because that way of stating it makes it pretty clear you yourself don't really see yourself as the author of the stories.
Simultaneously, you presented them to your friends as if you were the author. So basically your friends found out you BS'd them. Why wouldn't they be upset? Seems like a totally natural reaction.
Now, if you performed extensive labor to get the story into the shape it it's in, hours long conversations with the model, some manual tweaks, etc etc, maybe you have a better claim to authorship than you would if you used a one shot prompt (though your own framing makes it sound like you did very little)