I think it would be better to say: "This is not true in many scenarios, depending on how costs are derived"
Liabilities are very hard to determine.
We are not yet to the point of operationalizing much of this stuff very well.
Given the vast amount of latent energy in such things, it's probably worthwhile looking at how we can effectively streamline and enable safety, fully accepting the fact may be 'it's impossible'.
This is because most types of spent nuclear fuel slowly become "weapons-grade" as the short half-life isotopes decay.
Since Pu-240 has a shorter half life than Pu-239, it is technically true that the isotopic composition in spent fuel becomes more weapon-like as the fuel ages. But it is very slow because Pu-240 still has a half life in excess of 6500 years. It would take more than 13000 years for the decay of Pu-241 to improve the isotopic profile of today's power reactor plutonium to something near "weapons grade."