Indeed, and the waves were a heck of a lot taller than that. If you stacked them, would the power behind the water not just knock 'em flat? If it can demolish a sea wall, stacked containers won't last long.
I don't think that's how seawalls work. Those spectacular pictures of waves showed water being thrown upwards, not coming horizontally over the top. The bulk of the wave hits the wall lower down, and can only go upwards - so it pushes any water heading to overtop the wall upwards. Some seawalls are curved at the base to encourage this, but all do it.
Of course the water that shoots upwards has to come down, and water is very heavy. So the top of the wall takes a pounding, but from all directions. The Dawlsih wall was made of quite small blocks of masonry, mortared together - and mortar is not as strong material. So the wall can be taken down a block at a time. Big single lumps, if heavy enough, resist that. They might move a millimetre or so, but that's OK.
I think
NR» must have spent a lot of time remortaring blocks into the top of the wall, both after storms and routinely. Then along comes a storm big and strong enough to take the wall right down before any repair is possible. Once the embankment can be washed out, there's nothing behind the wall pushing back and it just gets pushed over.