Well, hydro energy is just there, requires comparatively little ongoing maintenance, and has less catastrophic failure modes.
I think the same caveats work for Dyson spheres.
Given incidence of dam bursts like the 1975 Banqiao dam failure[1], with an estimate death toll of 26,000 to 240,000 people and flooding of over 12,000 square kilometers, I'm inclined to disagree with this assessment.
Hypothetical scenarios can be constructed for both methods of power generation (What if Braidwood plant melts down, somehow killing the 5 million people living within a 50 mile radius? What if the Three Gorges Dam busts, inundating an area inhabited by 600 million people?[1]) Either way, it is far from obvious to me that hydro has the superior safety profile, especially when their fatality rates are on the same order of magnitude even when the Banqiao incident is removed[2]*.
[1] https://web.archive.org/web/20210620174812/https://www.japan... p/opinion/2020/09/01/commentary/world-commentary/big-china-disaster/
[2] https://ourworldindata.org/safest-sources-of-energy
* Sovacool et al. (2016), the source of the data in [2] includes a hydro fatality rate per tWh 2.4x larger than nuclear when Banqiao is included.