Duck and cover works, just as walking fast, or walking up hill at all works in this tsunami scenario, it will save many lives, but of course it won’t save those that it doesn’t, meaning those doomed in the 100% kill zone.
Duck and cover is not to save someone from being vaporized but to save the many from what happens if they stand there exposed. Many Japanese would have survived, and many would have avoided blindness and terrible injuries, if they had been trained to duck and cover.
The evidence that duck and cover would work in the WWII era is that some survivors were partially shielded and partially burned.
Up a hill in a tsunami works, up stairs or up a tree doesn’t work so well (even though it’s higher ground) when the wave hits if the tree or building do not survive (or the water in the building reaches the ceiling or the wave force knocks shelves down, knocks a person into a wall, etc.).
Ultimately when fleeing, you can only move as fast as those in front of you (and some do not have a good survival instinct) or if you can manage to get around them, somewhat faster but nowhere near “full speed”.