I'm sorry but that is not accurate. Water under pressure will still freeze but the freezing point will be lowered only slightly. The fact that the bottom of Lake Vostok is unfrozen is due entirely to geothermal activity..........An analogy would be why you have to dig fence poles at a certain depth below your prevailing frost lines.
As a side note, the deepest known ice lies in the antarctic and rests 2,555 meters below sea level, where the ice is over 4 kilometers thick.
It was at the Vostok station that the coldest temperature ever observed on Earth (−89 °C (−128 °F)) was recorded on 21 July 1983. [1] The average water temperature is calculated to be around −3 °C (27 °F); it remains liquid below the normal freezing point because of high pressure from the weight of the ice above it. Geothermal heat from the Earth's interior warms the bottom of the lake. The ice sheet itself insulates the lake from cold temperatures on the surface.Source: http://en.wikipedia.org/wiki/Lake_Vostok
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At the base of any substantial glacier, there will be liquid water simply due to the pressure, with or without geothermal warming. Same in the oceans where you can seem water temps below 0C at great depths. It does not have to do with frost lines... simply the pressure that changes the freezing point of water. Pressure also raises the boiling point.