Here is the entry for comma splice in Merriam-Webster's Eleventh Collegiate Dictionary (2003):
comma splice n (1924) : the unjustified use of a comma between coordinate main clauses not connected by a conjunction (as in "nobody goes there any more, it's boring")
This definition identifies two requirements for a comma splice: (1) it must involve a sentence that has coordinate main clauses (in MW's example, those clauses are "nobody goes there any more" and "it's boring"); and (2) the sentence must not use a conjunction (such as and, or, or but) to connect the coordinate main clauses.
Because your example doesn't have a conjunction between "The more important idea belongs in the main clause" and "the less important in the clause introduced by the subordinate conjunction," it might appear to satisfy the second criterion I just listed. But because the sentence doesn't satisfy the first criterion—since "the less important in the clause introduced by the subordinate conjunction" isn't a "main clause" capable of standing by itself and making sense as intended without borrowing the verb belongs from the first clause of the sentence—it actually doesn't satisfy the second criterion either.
Your example sentence is a parallel construction with a shared verb (belongs) and a shared noun (idea) that appear explicitly in the first clause but implicitly in the second clause. If you reproduced the sentence with those elements explicitly stated, you'd have this:
The more important idea belongs in the main clause, the less important idea belongs in the clause introduced by the subordinate conjunction.
And then you would have an instance of a comma splice—which you could avoid either by changing "clause, the" to "clause. The" or by adding a conjunction after the comma: "clause, and the." But because it omits "idea belongs" from the second clause, the original sentence makes the second clause depend on the first clause for its coherence.