Is it? There aren't many useful details in what I read so far, I'm missing clarity on issues such as: what exactly (what kinds of physical evidence and/or simulation) is the basis of the new meltdown timeline estimate, what is the amount (even just the order of magnitude) of fuel that melted down and collected at the bottom of the pressure vessel, what is the amount of corium that escaped the pressure vessel after melting it through, what is the current estimated temperature of both pools? For your worse-of-the-worst scenario, material equivalent of that in multiple fuel rods would have had to collect in a single pool. If it is in smaller amounts, then just decay heat will have kept it hot until the flooding of the core, but there was no chain reaction.

Either way, if the bottoms of the cores got dry and there was lots of corium on it, they might have been lucky not to produce a major team explosion during the flooding.

_{*Lunatic*, n.
One whose delusions are out of fashion.}