Should be mentioned as well that a velocity of 0.5-0.8 km/s for the observed H2O gas expansion is nothing to get one's knickers in a twist about either. Previous observations and, more importantly, modeling, suggest that is an expected expansion rate for H2O on a comet, where it is escaping into a vacuum. There are various processes that control the velocity, and it is all rather involved, and many of the papers that seem to be quoted regarding the modeling, no longer seem to be easily available.
Suffice to say, the velocity is not unexpected, and doesn't require "pressurised jets".
To quote from one paper I read online.........."This value of 0.5-0.6 km s^-1 is signifcantly below that predicted by thermo-hydrodynamical models. Those models show that the gas outflow velocity at the distance where H20 molecules dissociate is at least 0.7-0.8 km s^-1."
From
http://articles.adsabs.harvard.edu/...=NO&data_type=GIF&type=SCREEN_GIF&classic=YES
So really just another argument borne of ignorance of the science. These models have been around for a long time. Predates the internet, which is probably why they are difficult to find.