I think the arguments for this would possibly be, if AI continues to be useful (generation demand skyrockets): Meta would possibly have a positive ROI for these investments which would lead to others copying the investment strategy and building more nuclear. If that happens a large portion of AI demand would become green(-ish) energy.

If AI demand lowers (generation demand plummets): Meta would have subsidized a bunch of nuclear reactors which would likely continue to produce power for 10 years - 50 years.

A big reason I have heard for lack of nuclear build out is the lack of starting capital but after they are built they are generally stable and maintenance is predictable.

An example of this: https://en.wikipedia.org/wiki/Beznau_Nuclear_Power_Plant. It may be turned down eventually but a 60 year runtime is pretty impressive for 60s engineering!