Haven't played this in many years but checking rules link here it seems my memory is right. You dont 'gain' coins in the same way you get trade or production. Your coin dial is just showing how many coins you have in total. If you read the bit after what you quoted it should be clearer.
Coins gained from squares on the map – whether from a natural
gold deposit in the mountains or from a building placed in a
square – are only kept as long as the player retains control of the
square (i.e., the square is contained in the outskirts of one of the
player’s cities or one of the player’s scouts remains in the square
to gather from it). If an enemy figure blockades the square
containing the coin, it is lost while the enemy figure remains
there (see page 27). Similarly, if a building granting a coin to a
player is destroyed, the coin is lost.
So if I move a scout on on to a space giving me a coin I move my dial up straight away. if I lose that space then I lower my coin dial. Its not a resource thats gained every turn and goes up each time. if it was a player with 3 coins on tech and city outskirts would win in just a a few turns.