The fungus they use is already widespread in nature. Their "version" is probably less well adapted to nature than the "original", and should be out-competed by

The fungus they use is already widespread in nature. Their "version" is probably less well adapted to nature than the "original", and should be out-competed by the wildtype at worst. But if I understand the article correctly, the fungus is quite deadly to mosquitos, achieving "local" extinction quickly, which also stops further spreading of the fungus.