In chapter 11, "No Pressure though":

and the back side is the only one that can accept Liquids

The back side is the only one that can accept Liquids by default.

Either I've missed it, or the questbook doesn't include information about Mekanism machine IO:
All Mekanism machines (including gas tanks etc.) can be configured so that each side¹ can input or output items², liquids, energy and/or gasses (as applicable for the machine, and not both input and output at the same time)
Initially, the side IO interface shows only items, but there are tabs on the sides to manage the other materials.
The edge of each element in the machine UI determines which input/output setting it is affected by.

In the case of the PRC, the default setting for liquid input (yellow edge, liquid tab) is indeed set to back side only, but that can be changed as needed.

EDIT: Modpack version is 2.9.1, plus some other mods not relevant to the issue.

¹: Unlike Thermal Expansion machines, the front face of Mekanism machines can be used for IO.
²: "Special" (purple) is used for Metallurgic Infuser "reactive" materials such as redstone, diamonds and refined obsidian.