To be even more clear, a steam explosion cannot by any stretch of the imagination "level" a city fully two hours away. And the phrase "equivalent of a 5 megaton explosion" has no possible interpretation that squares with reality. The idea of a megaton-sized steam explosion is ludicrous, so I assume he means to imply some connection to radioactive fallout - but the amount of fallout isn't really correlated with the strength of the explosion. In fact, early fission bombs were relatively dirty while later thermonuclear bombs were relatively clean. The largest nuclear explosion ever produced was actually remarkably clean due to its unique design. So what is Moxie even talking about? It borders on incoherent, and I think you're being far too charitable.

By the way, the three men who faced "almost certain death" were certainly heroic but weren't in nearly as much danger as he plays up. All three men survived and lived for decades afterward.