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.