Does an ETB effect trigger it's own card's triggered ability?

See title, for example [[Generous Patron]]. It's a long running question I had with a friend, and we kind of came to the conclusion that the answer is no because the creature doesn't yet exist when the ETB resolves.

But I gave it some thought again because it feels so weird that so many cards have this kind of effect and supposedly it doesn't get triggered. Does anyone have a definitive answer to this? Would be great if you can refer to the official rulings.