No, that is incorrect, though it has often been repeated by less technically informed advocates of the thorium fuel cycle. The vast majority of the world's commercial nuclear reactors are light-water-moderated pressurized water reactors or boiling water reactors (PWR and BWR respectively). The
actual military origins of these light water reactors is that they were originally researched for submarine and ship propulsion for the US Navy:
https://en.wikipedia.org/wiki/Light-water_reactor
https://en.wikipedia.org/wiki/Pressurized_water_reactor#Hist...
https://en.wikipedia.org/wiki/Boiling_water_reactor#Evolutio...
The United States and other nuclear weapons states never produced their weapons plutonium from light water reactors. Weapons grade plutonium has typically been produced from uranium in dedicated plutonium-production reactors moderated with heavy water or graphite. (The United States once had a single reactor that also produced commercial electricity along with weapons plutonium, https://en.wikipedia.org/wiki/N-Reactor, but it wasn't a light water reactor either.)
Since uranium fueled light water reactors weren't used to make plutonium for weapons in the first place, the thorium-myth claim that commercial power reactors stuck with uranium so as to make weapons material is ridiculous. The military's early sponsorship of light water reactors for naval propulsion did give uranium-fueled LWRs a significant historical path dependency advantage. I speculate that thorium advocates who continue to repeat the myth about how uranium LWRs became dominant do so because it makes the dominant technology's dominance sound more sinister, hence thorium sound more attractive by contrast.