the pressure inside the straw is not the reason that something would float up inside the straw.
it is the density differential that would cause a object to float up inside the straw.
the water in the straw has no pressure in any direction.
at the top of the straw the plug is supporting all the weight of the air and water in the straw.
the weight of the water in the straw is causing a vacume in the air between the plug and the water.
the water in the straw is trying to fall , but cannot because water will not stretch , so its just hanging
there exerting no pressures at all.
picture a board that is nailed to the ceiling.
the board would fall if it were not supported by the nails.
the nails in the straw example is the vacuum in the straw.
the plug is the ceiling.
the water is the board.