Occam's Razor says this is the most likely answer. I tend to expect these boring machines are capable of going through solid igneous rock, albeit at a much slower rate.
They never would have started the project unless their equipment had the capability of grinding its was through igneous rock. That's what makes this story so intriguing.
What could they possibly have run into that's too hard for Bertha to grind through? Or is it that they don't want to damage whatever it is they've found?