Thats not the point. To design ,build and operate this plant you first need to use a lot of energy, tools and resources that are not carbon neutral. Before this plant is even operational it will be carbon negative.
Interestingly if they produced everything used in the plant local to Iceland, at least in terms of energy used it would basically be 100% geothermal sourced.
But they won't be able to. Industrial controllers, motors, computers are not made in Iceland. These items alone need a lot of CO2 to be made.