I don't think a massive expansion of nuclear power is the only way. Sure it might play a role but why would it be the only way. Look at this study for example.
It doesn't have to be the only solution. But any proposed solution that does not include, in addition to some expansion of wind and solar energy production, an immediate and massive nuclear expansion is suspicious to me when it comes from somebody who purports to simultaneously believe in one or more of the apocalyptic climate scenarios.