This is not describing the entire mountain, it's describing a "part of the mountain-side". There is no way of knowing what shape the entire mountain is based on this excerpt.
Vertical faces can still be smooth and flat. It means that there are no bumps.
Flat does not require the surface to be horizontal.
A wall, for example is very flat but it is still vertical.
So, imagine this "part of the mountain-side" to be a wall, and that should help you understand what the author is describing.
As to the first sentence, remember that many (or most) bugs can climb vertical surfaces due to having specialized feet. In general, most animals don't have this ability.
Birds could easily reach this point by "climbing" through the air ... in other words, by flying.
Monkeys are well known for their climbing abilities... though I'm not really sure that, if the description is to be believed, even a monkey could manage it.
The point of the two sentences is to imply that this is a very steep, vertical walk for the beetle and not many other creatures could manage it without special abilities like flight... but for the beetle, it is a simple matter of a walk... not a struggle at all.