The authors attribute this to the lack of unbound water in rocks. The neocatastrophist scenario opens many avenues for tectonic disturbance of rocks, creating channels where none existed before. Even without catastrophism, I do not consider that a batholith can be emplaced without considerable disruption to the country rocks.
Shales have permeabilities of the order of a few millidarcies.
Furthermore, when people start to look for evidences of convection, they are likely to find them. The shapes of the metamorphic aureoles were inconsistent with purely convective heat loss, and yet could be explained by invoking convective activity. The first sentence is again model-dependent. To produce a realistic model that cools all batholiths within a year is hard. In addition, I am sympathetic with the expanding earth concept - which has thermodynamic implications which favour rapid cooling.