it has been plausibly argued to me that all the roads to nuclear weapons, including plutonium production from U-238, may have bottlenecked through the presence of significant amounts of Earthly U235 (apparently even the giant heap of unrefined uranium bricks in Chicago Pile 1 was, functionally, empty space with a scattering of U235 dust).

All is such a strong word unless supplemented with qualifiers. I question the plausibility the arguments at supporting that absolute. The route "wait for an extra century or two of particle physics research and spend a few trillion producing the initial seed stock" would still be available.