As has been stated, the size and composition of the ork forces in this adventure are confused and contradictory.
At one point, it is stated that there are 8 boyz, 1 nob, a runtherd and 5 gretchen defending the refinery. Two sentences later, there are 30 boyz, a nob, runtherd and perhaps 5 gretchen. This is the small and weak defense force the PCs are meant to oust with a minimum of fuss?
The frightening attack force that must hold off at any cost however, consists of 1 big mek, 1 weirdboy, 2 nobs, 10 boyz, 10 gretchen, a looted hellhound and an undisclosed number of wartraks. While the vehicle(s) and the weirdboy are intimidating, there are far fewer boyz in this attacking force than in the "relatively small*" defenders.
* quoting the adventure summery on page 248.
In the end, I am forced to wonder how many orks are really supposed to be in this adventure… keeping in mind that they should be handled by about 10 guardsmen, likely 4-5 PCs and the rest comrades, outfitted with their personal kit, a chimera, a heavy bolter and some demo charges.