While 5 megatons definitely does seem like an overestimation, I think the fear was that it was a much larger mass of molten corium, including not just the fuel

While 5 megatons definitely does seem like an overestimation, I think the fear was that it was a much larger mass of molten corium, including not just the fuel itself, but also molten silicon and lead, from the material dumped on the exposed reactor to quench the fires and prevent fallout. There was an estimated 5000 tons of this suppressant material, although I don't know how much could have reasonably been expected to melt (and I think later analysis shows that very little of it actually landed on the exposed core itself.) Then when the molten corium hits the water, the water could act as a neutron moderator for a runaway fission reaction. You have to think this would be more a nuclear "fizzle" than the prompt criticality that is necessary for a >10kt (let alone megaton) bomb, but it would still be bad. A 5 megaton explosion that levels Kiev? Probably not, but then again I'm not the nuclear physicist who came up with that number. The idea that blowing the core material into the atmosphere and causing further meltdowns in the other three reactors would be a major radiological disaster for Northern Europe, though, I can appreciate.