This is an example of a temporal cum clause (as opposed to a causal or concessive cum clause), in which the verb within the clause may take either the indicative or the subjunctive.
When the verb within the cum clause is the imperfect or pluperfect subjunctive, it should be taken in a descriptive sense.. This is explained in Latin Grammar by Allen and Greenough:
546. A temporal clause with cum and the imperfect or pluperfect subjunctive describes the circumstances that accompanied
or preceded the action of the main verb.
That being the case, a distinction has to be made, because the indicative can also occur with these tenses (although your example is actually with the perfect tense). The difference is that the indicative is used in a definitive sense:
545. A temporal clause with cum (when) and some past tense of the indicative dates or defines the time at which the action of
the main verb occurred.
Another way to put it is to say that the indicative, in this case, answers the question 'When?', that is, it defines a specific time when the action of the main clause took place. In terms of your sentence: When did he return to Rome? When he heard about Caesar…
The subjunctive, on the other hand, serves to describe the circumstances under which something occured, without the intention of specifying a specific time. For example:
Cum id nuntiatum esset, Romam rediit.
When this had been reported, he returned to Rome.
Since the subjunctive is used, you can conclude that the purpose of the clause isn't to define the time, but merely to describe the circumstances. He returned to Rome (at some unspecified time) when the news was already out. The purpose may have been to indicate that he also had knowledge of the report, but it wasn't intended to specify the time when his return took place.
