His weapon didnt actually destroy an island,it just destroyed all life at it with the poison gas.He just found a really effective poison gas and during that accident too much of it was released.
How does that come close to destroying the world.Check a random sland in the world map to see how small it actually is.Oh right,you cant,unless an island is really hue,you wont even see t.Thats the difference between destroying the eco-system ofd an island and actually destroying the world.
And like aggeroff explained those things dont progress as fast as you think.

