Local Diaspora affect only single land.

Local Diaspora is only event that doesn't scale at all with number of players/boards. Whether 1 player or 4 players, the text as is affects only 1 land on the island.

Is that a typo/mistake? Or if not, I'm curious why this one event is different?

There has been word in the past, so I would assume that it is intentional. As for why, I would guess it's because the results of it on one board could change which land it applies to on another board.

I might also guess that in a larger game, the single land with the most Invaders will tend to be more built-up.