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.

The largest ever fission explosion was, IIRC, something like a few hundred kT. It's just physically impossible to have a bigger yield from fission alone, as the bomb assembly blows apart before it has time to fission. And this was from a carefully designed weapon, not a reactor fueled with LEU melting. So yes, 5 MT is pure fantasy with absolutely no connection to reality.

As for prompt criticality, in a weapon the neutron multiplication time is around a million times faster than in a prompt criticality transient in a thermal neutron reactor. That's what allows a weapon to have any significant yield before it disintegrates.

The largest fission device tested was the 720Kt UK Orange Herald:

https://en.wikipedia.org/wiki/Orange_Herald

As Wikipedia says, that was a boosted fission device, not pure fission.

I should probably have said "single stage" rather than fission!

This is the worst case scenario, right?

The worst case scenario is still orders of magnitude less than "5 megaton explosion". Somebody's math was wrong, or lost in translation. Either the math was done wrong, not done at all, or the figures got messed up in translation.