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.
See this reddit thread[0] linked elsewhere here. The issue is likely a faulty primary source, some russian official who wildly misspoke or miswrote something.
> but the amount of fallout isn't really correlated with the strength of the explosion.
It absolutely is for a similar burst condition and similar weapon technology, so it's absolutely credible that one would use, say, groundburst yield with a technology then dominant for large weapons as a reference for fallout. (And, IIRC, groundburst yield is both much greater and much less variable by nuclear bomb technology than airburst since most of the fallout is ground material with induced radioactivity while with an airburst most of the fallout is bomb material, so that technology might not even need to be significant variable using groundburst yield as a reference scale.)
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.