The sentences are different because so and insofar as are different types of word. So is a proform adverb. In sentence (1) it means a modern phenomenon. When this type of so is moved to the front of the clause, we have to change the position of the Subject and the auxiliary:
- Mary is hungry. So is my elephant.
In the example above so means hungry. The second sentence means My elephant is hungry too. We can see that the Subject my elephant and the auxiliary verb is have changed places.
Here is the Original Poster's first sentence:
- Just as the nation-state is a modern phenomenon, so is the explosive increase in the human population.
The proform so in this sentence means a modern phenomenon. The subject of the second clause there is the explosive increase in the human population, but it appears after and not before the auxiliary verb. We do not need a second Subject, it, here. We already have one!
The second example is completely different:
- ... Marxism is strong insofar as it is true.
Here the phrase insofar as can be analysed as a compound preposition. It takes as a Complement a declarative clause. In the sentence the pronoun it refers to Marxism. It comes before the verb is because there is no special reason to move it from the normal Subject position. We cannot drop it because every tensed verb in English must have a Subject.