But they have back up diesel generators and backup coolant pumps right?

Correct.  They have both.  What is more at issue is the height at which the wiring to coolant pumps exists.  Because wet wires will not be able to power the pumps, regardless of whether the diesel generators are producing electricity.
    This was the end lesson and fatal flaw of Fukushima.  Once they had the diesel gas and generators it didn’t matter because the “switchgear rooms” which are just giant breaker panels for all the wiring, were too low and were swamped with ocean water.