The rulebook doesn't say what you remember it saying.

It specifies exactly the opposite:

2.b. Build a Stage of a Wonder

To build each stage of his or her Wonder, the player will use a card of their choice as a construction marker:
To do that, the player must pay the price shown on the Wonder board and not the one shown on the structure’s card. The player then plays the card, face down, half hidden under the Wonder board to show that this stage of their Wonder is now built. The card then has no other effect, and is not considered to be a structure.

EDIT: The rulebook doesn't say what the Croatian translation may imply.

It specifies exactly the opposite:

2.b. Build a Stage of a Wonder

To build each stage of his or her Wonder, the player will use a card of their choice as a construction marker:
To do that, the player must pay the price shown on the Wonder board and not the one shown on the structure’s card. The player then plays the card, face down, half hidden under the Wonder board to show that this stage of their Wonder is now built. The card then has no other effect, and is not considered to be a structure.