I don’t think they were saying that just the resources to build the reactors would exceed that CO2 budget, but rather that the world would exceed that CO2 budget in the time it takes to build the reactors.
Would be interesting to estimate and compare nuclear vs alternative "initial co2 efficiency per 1 megawatt priduced", considering large lifespan and power of reactor.
I'm unsure of the CO2 quantities so I cant make a statement as to whether its worth the CO2 cost.
What I can say is that wind, solar and other renewable alternatives also have initial CO2 costs, possibly lower than that of nuclear reactors.