But it isn't, as the OP says most of the water is removed and replaced with pebbles, how can the water column then be the same height?
My ice tea is less than half a glass without the ice.
The water column can be a mile hole or a bunch of soda straws, as long as the column height is the same the pressure is the same.
Same goes for wet sand, at least until the space between particles becomes small enough for other effects.
This is water pressure. the pressure of the pebbles is much higher.