Is “so little is clear” in “So little is clear that Castro’s departure took on the quality of a test...” an inversion?

There are two obvious ways to parse that sentence. You could use "so" as a conjunction:

So... (little) is clear that Castro's departure took on the quality of test case for the incoming leader of the new world.

Or you could use it as a modifier to "little" to say how little:

(So little) is clear that Castro's departure took on the quality of test case for the incoming leader of the new world.

Either way, 'little' is technically the subject.

Side note: the phrasing here feels very awkward to me. Usually the phrase "little is clear" is used to show that something is unknown. Like: "Little is clear about why the vehicle took a sudden turn..." or "Little is clear from the meaning of this sentence..." Using that after it ("Little is clear that...") is weird. I'm not quite sure what the author was going for here.

Edit to add...

I think @JohnLawler's suggested paraphrases in the comment on OP's original question highlight the two alternative interpretations well:

(1) Little is clear -- so little that Castro’s departure took on the quality of test case. [so as modifier]

(2) Little is clear, so that Castro’s departure took on the quality of test case. [so as conjunction]

I think the original author probably meant #1, but I still think it would have been better had it been rephrased.

I suspect you are being confused by the odd verb tenses.

This sentence is at odds with what we expect from a form like "It is clear that it took ..." In the OP's example sentence, the shift in tense is needed to connect the examples rendered in present tense to the past event. But logically, the present can't be used to explain why something was viewed that way in the past. It can be used to justify it, but it can't be said to be "how" it came about. This may be why you sensed that somehow, something got inverted. It did - logic got inverted - and journalist are famous for doing this. But this sentence doesn't invert syntax the way you suggest.

What the writer has left unsaid is that the unsettled geopolitical landscape of which he gave current examples has existed for a long time.

So little (of the geopolitical landscape) has been clear that Castro’s departure took on the quality of test case for the incoming leader of the new world.

The writer is perfectly silent regarding who made this assignment, why we should accept it, and what the "incoming leader of the new world" thinks about it. But they seem to be suggesting it was inevitable, which, of course, it wasn't.