He didn't say other architectures would be no good, he said they're less likely to be safe.

He thinks the distribution P(Outcome | do(complete Oracle AI project)) isn't as highly peaked at Weirdtopia as P(outcome | do(complete FAI)); Oracle AI puts more weight on regions like "Lifeless universe", "Eternal Torture", "Rainbows and Slow Death", and "Failed Utopia".

However, "Complete FAI" isn't an actionable procedure, so he examines the chance of completion conditional on different actions he can take. "Not worth pursuing because non-implementable" means that available FAI supporting actions don't have a reasonable chance of producing friendly AI, which discounts the peak in the conditional outcome distribution at valuable futures relative to do(complete FAI). And supposedly he has some other available oracle AI supporting strategy which fares better.

Eating a sandwich isn't as cool as building an interstellar society with wormholes for transportation, but I'm still going to make a sandwich for lunch, because it's going to work and maybe be okay-ish.