your link does not include nuclear, it only includes an elusive "advanced nuclear". This is not for China, which I have the feeling has a different take on "advanced". In the US, existing plants have much lower costs, around the same as cheap coal and gas.
PV capacity costs about 1 dollars per Watt and achieves a 20-25% capacity factor. In the article they say less than 0.5 dollars per watt of nuclear. If they can achieve that, it's obviously cheaper to go nuclear than PV. Nuclear has higher operating costs, true, but also a 90% capacity factor, meaning you need 5 times less capacity installed than PV. 1200 GW of nuclear for 440 billions is the equivalent of 5000GW of PV for 5000 billions. This is just to produce the same amount of energy, at any time during the day/year, before considering the cost of storage or intermittent power on the grid.
I truly hope China can pull this off at the advertised price. Because of the sheer size of the initiative they might be able to just do it using the economy of scale. France did it in 10 years in the 70s-80s, no reason China can't do it today.
We need all the energy sources, being PV+wind+battery purists won't be enough.
I found this article with a more global take https://world-nuclear.org/information-library/economic-aspec...
Apparently France managed to build it's nuclear plants at a cost of 1 dollar per Watt. Going to 0.5 dollars today might be a challenge for China, even with economies of scale. The article estimated 2.5 per Watt.
Even at 2.5 USD per Watt installed, that would still be about the same price as PV installed. But you gain stability in the grid for a slighter higher operating cost.