Maybe this statement is just a reflection of what can be done in a reasonable amount of time (a few years)? Planning and building nuclear plants takes an order of magnitude longer, it seems.
If nuclear were the only option, it would be different. But the only pro-nuclear policies that make sense for achieving 1.5-2 degrees are keeping existing plants online and reactivating old ones (also policies with reasonable disagreements, but at least positive regarding climate change afaict). Building new ones simply takes too long to achieve climate goals while wind & PV are reasonably quick to do (if the political will is there).
How I wish that where true. But looking at all current new Nuclear power plant projects in Europe and the US it's clear that we are incapable of that. Yes we can argue about why that is and how in theory it shouldn't take that long, but the truth is that best case scenario it will take you more than 5 years just to get a building permit to start buildings a new nuclear power plant.