Actually I found my answer myself when writing another thread. I noticed that the Big Mek is lying in wait 200 meters away from the other side of the wall.
Which then raises another question. How do they shoot -anything- through 150 meters of Deathworld Jungle???
Even if he tries to shoot indirectly, he still has to deal with the Jungle canopy, and would be making a completely blind salvo of shots at the wall.
Wouldn't this be more than just a little likely to miss and hit the refinery (which the BM doesn't want to do) as the tanks are less than 100 meters beyond the wall?
Furthermore, how does he know if his men have made it into the base or not? I doubt he can see through 150 meters of Deathworld Jungle.
Then finally, he can't exactly race into battle through that jungle, even on a war trakk. It would take a good 10 to 15 minutes to drive anything 150 meters through jungle unless it was along a road or trail. (e.g. he was going for the front gate) .
Perhaps he and his men should sneak up to the tree line as the hellhound attacks and then fire their salvo and charge. Then have him race the 50 meters across the clearing.