It absolutely can and does create a seal strong enough to keep the gas contained. Note the inside of the bottle cap, it has a rubber gasket which is pressed against the lip of the bottle mouth to create a seal. Furthermore, this seal doesn't have to resist as much pressure as you might think, most of the carbon dioxide is dissolved in the liquid, the cap only has to contain the pressure in the small head space enough to shift the equilibrium toward dissolved CO2 over gaseous. Same as with soda bottles.