The seals don't have to be uncompressible, though.

And the amphoras themselves are ceramic and might not be completely tight at those pressures, meaning sea water might be able to get in, and eventually replace the wine. 2200 years is a long time for slow processes to work through ancient clay vessels.

Look at a (glass) wine bottle - there are a few centimeters of ever-so-compressible air between the top of the wine and the cork. If the Roman wineries did not leave a similar bit of air in their amphorae, then their seals would need to be quite flexible - think of a thermometer bulb, and Rome's lack of controlled-temperature shipping technology.