The turbine was discharging steam into a seawater-cooled condenser which would instantly lower the steam pressure on that side, that is, it would create a partial vacuum. It is the difference between the (relatively) high pressure inlet side and the (relatively) low pressure exhaust side that gives the energy to the blades. It is a closed system where outside atmospheric pressure is irrelevant, and if they said absolute they probably meant it. Cheers.