The campaign finance violation where Trump is unindicted co-conspirator is not part of the Mueller Russia probe even though Cohen's lying over Trump Tower Moscow was also involved in his case. Unlikely the campaign finance matter is in the Mueller report. Seems that there was recent reporting that SDNY is still continuing that investigation however.

I'm guessing another prosecutor will (or should) take it up.

And remember, Mueller didn't name Trump as Individual 1. Those were Cohen's words and so it's Cohen's claim that Trump did certain things. Mueller didn't pass judgment except to include Cohen's testimony as evidence in his plea deal that led to sentencing. No corroboration was necessary; it wasn't an officer of the court taking responsibility for the words. The only person at issue was Cohen, in Cohen's, um, "defense" isn't the right word here.

Decades ago it was the prosecutor who mentioned "somebody" as a nameless individual, with a muted allegation of complicity in wrong-doing. That places it in a (slightly) different category.