And more to the point, nobody waited for nukes. The allies tackled the problem with the tools they had available at the time, and made use of the nukes when they were ready.
edit: the comparable timeline would be "currently approved power plants that are planning construction could put the brakes on climate change", I mean, if it were true. Because deadlines for substantial impact on climate change are "next few years, decade or two tops", and fusion will be ready far too late.