Meaning of "a siren calling attention to a fog that it does nothing to dispel!"

Your understanding corresponds to what is usually meant by this simile. A confirmation can be found here.

• To paraphrase : 'scholars identify and talk about difficult things but don't explain them'. A siren warns ships of fog but does not make it go away.

ADDITION

Here are further points of interest concerning this simile; motivated by questions raised in the comments, I'm adding them to the answer. What is being asked is not an explanation of the logic in this comparison but what it means. That being said, this phrase is acknowledged as a definition of a scholar by John Nash Douglas Bush (1896–1983), a literary critic and literary historian from the past century (definition of a scholar, Wikipedia).
This definition, although of recent origin, is considered by some people as old (pejoratively so) and waggish, not serious, as it is a critique of scholarship (opinion).

On the question of this definition by simile being aptly made or not, let's say that it seems correct to consider it as wanting in the way of having minimal qualities of identification. As has been said in the comments the foghorns are made to warn of dangers in the fog, embarcations or rocks hidden by the fog, that is. It is only necessary to determine a course for the boat that makes for decreasing sound intensity to ward off the risk. So what the siren does is to call attention to a danger not saying how to keep safe from that danger. To explain the danger is irrelevant, to make it disappear (dispel it) is irrelevant too. As far as the parallel scholar's activity goes "to call attention" to a subject could in the extreme be assimilated to "describing a subject"; the subject is then a fog, which won't do: the idea of dispelling the subject has no consitency. If, instead, the opaquenes of the subject is assimilated to a fog, wrapping it up, then "dispelling the opaqueness" is acceptable, but calling attention to the opaqueness, in other words "describing the opaqueness" does not correspond well. What is important is the meaning of "scholars identify and talk about difficult things but don't explain them", the paraphrase; the attempt at a perfect correspondence in the explanation, such as "A siren warns ships of fog but does not make it go away." is bound to fail and is of not much consequence. The meaning in the paraphrase is expressed in a slightly different form that seems little known: "Science describes everything and explains nothing.". This conveys rather faithfully what Douglass Bush intended to say by means of his comparison.