I hadn't thought of this dichotomy before, but I'm not sure it's going to be true for long; I wouldn't be surprised if it turned out that obtaining the 50k H100s you need to train a GPT-5 (or whatever hardware investment it is) is harder for Iran than obtaining its centrifuges. If it's not true now, I expect it to be true within a hardware generation or two. (The US already has >=A100 embargoes on China, and I'd expect that to be strengthened to apply to Iran if it doesn't already, at least if they demonstrated any military interest in AI technology.)
Also, I don't think nuclear tech is an example against obfuscation; how many countries know how to make thermonuclear warheads? Seems to me that the obfuscation regime has been very effective, though certainly not perfect. It's backed with the carrot and stick of diplomacy and sanctions of course, but that same approach would also have to be used if you wanted to globally ban or restrict AI beyond a certain capability level.