Multiple countries have detonated nuclear bombs using U-233 derived from thorium reactors! [0] Practically I agree with you that thorium is proliferation resistant and if someone is bomb hungry they won't prioritize it, but if you want to set up the bomb and all you have is thorium... The infrastructure wouldn't necessarily be significantly larger or worse than conventional enrichment.

0 - https://en.wikipedia.org/wiki/Uranium-233