I could claim "nuclear weapons are possible" in year 1940 without having a concrete plan on how to get there. Just "we'd need a lot of U235 and we need to set it off", with no roadmap: no "how much uranium to get", "how to actually get it", or "how to get the reaction going". Based entirely on what advanced physics knowledge I could have had back then, without having future knowledge or access to cutting edge classified research.

Would not having a complete foolproof step by step plan to obtaining a nuclear bomb somehow make me wrong then?

The so-called "plan" is simply "fund the R&D, and one of the R&D teams will eventually figure it out, and if not, then, at least some of the resources we poured into it would be reusable elsewhere". Because LLMs are already quite useful - and there's no pathway to getting or utilizing AGI that doesn't involve a lot of compute to throw at the problem.

I think you're falling victim to survivorship bias there, or something like it.

In 1940 I might have said "fusion power is possible" based entirely on what advanced psychics knowledge I had. And I would have been correct, according to the laws of physics it is possible. We still don't have it though. When watching Neil Armstrong walk on the moon I might have said "moon colonies are possible", and I'd have been right there too. And yet...

In the case of nuclear weapons, we had a theory that said they were possible. We don't have a theory that says AGI or ASI is possible. It's a big difference.