Yes nothing is failsafe 100% But remember that these reactors have several billion dollars invested in capital in them - so building robust shutdown should be possible
I think the idea behind passive safety is keeping it simple & not reliant on complex systems e.g just reliant on gravity for instance (e.g when power is gone gravity pulls the fuel rods out) plus redundancy so that if one system fails 2 or 3 others will do the job.
IMO my undestanding is their systems were not very robust. I am not a reactor designer however I am amazed that they failed & the fact that 2 reactors failed indicates to me that they were fragile.
