The thrust of it is nuclear power takes long enough to build that if we try to meet the 1.5C goal using principally nuclear power by the time the plants come online we will have put enough carbon into the air from the existing power sources that we'll blow past 1.5C and get into the 'absolutely catastrophic sea level rise' territory instead of just causing whole island nations to disappear and the largest displacement in human history we're currently aiming for.
I think it amounts to "this only solves half the problem, so we shouldn't do it".
Most big problems aren't solved by just doing one thing. When global warming is finally solved, it will have been by 10 separate things that each solved 5-20% of the problem.
Also, imposing an arbitrary deadline in 2029 is horrible project management. In a commercial project it's also dumb, but at least there you can cancel the project, and people move on to do other things.
For Earth, we can't cancel the planet in 2029 if targets weren't met.
