At some point you have to choose an arbitrary level to design to. I imagine this plant was designed to withstand a tsunami of level Y (100 year tsunami?) and this was just one of level Y+epsilon.
Yes. However, it's problematic to set such a low design threshold (100-year event) when the facility will be housing radioactive waste capable of poisoning the environment for generations (1000-year event). It seems like we need much more robust design for this type of critical infrastructure if we intend to use it.
(I don' mean to suggest you were saying this was an appropriate design threshold.)