Where it came from
The story.
Every messenger delivers instantly, anywhere. That is the whole point — and it is also what makes a message weightless. A note that arrives in half a second at any spot on earth costs nothing to send and nothing to read.
Yomble puts one condition back in: the place. A message is written at a spot on the map and stays there. Whoever receives the link has to be at that spot to read it. Not a filter, not a setting — a condition that cannot be tapped away.
The rest follows from that. There are no accounts, because a place is enough to address someone. The server never learns where a message is, because the coordinates travel inside the link, in the part a browser doesn't send. And the access code goes separately, so a leaked link on its own gives nothing away.
A message you have to go somewhere for is worth more than one you get for free.