How do Calvinists explain Calvin's use of Baruch? In his commentary of 1 Corinthians,  Calvin seems to use Baruch to explain verse 20 of chapter 10,


  It is certain from the Prophet Baruch, (4:7,) that those things that are sacrificed to idols are sacrificed to devils (Deuteronomy 32:17; Psalm 96:5.) In that passage in the writings of the Prophet, the Greek translation, which was at that time in common use, has δαιμόνια — demons, and this is its common use in Scripture. How much more likely is it then, that Paul borrowed what he says from the Prophet, to express the enormity of the evil, than that, speaking after the manner of the heathen, he extenuated what he was desirous to hold up to utter execration!


Here I seem to see Calvin confirming the prophetic office of Baruch. Confirming the prophetic authorship of the book of Baruch. And confirming the apostle Paul's use of Baruch.

How would a Calvinist today explain his words?
Actually, in his magnum opus &quot;Institutes of the Christian Religion&quot; (ICR), he cited Tobit, Baruch, 1 and 2 Maccabees, and Siriach. The Geneva Bible, which Calvin accepted and fostered, also contains the Deuterocanonical books. This doesn't say however that Calvin accepted them as having the same rank as others books. In fact, he wrote precisely about this.
First, to give context, recall that ICR was written in 1536. The Council of Trent, in response to the reformation wave, was held between 1545 and 1563. Among all the conclusions of the council, the fourth session (1556) issued a decree, with full doctrinal power, about the Canon of the Bible, reasserting the inclusion of the Deuterocanonical books.
In response to this decree (and every other decree of the council), Calvin wrote &quot;Acts of the Council of Trent, with the Antidote&quot; (albeit published posthumously, in 1547, it seems). The specific section about the biblical canon is quite long, but I present some extracts below (emphasis mine):

Secondly, in forming a catalogue of Scripture, they [the Synod of Trent]  mark all the books with the same chalk, and insist on placing the Apocrypha in the same rank with the others. [...]
Add to this, that they provide themselves with new supports when they give full authority to the Apocryphal books. Out of the second of the Maccabees they will prove Purgatory and the worship of saints; out of Tobit satisfactions, exorcisms, and what not. From Ecclesiasticus they will borrow not a little. [...]
I am not one of those, however, who would entirely disapprove the reading of those books; but in giving them in authority which they never before possessed, what end was sought but just to have the use of spurious paint in coloring their errors? [...]
Of their admitting all the Books promiscuously into the Canon, I say nothing more than it is done against the consent of the primitive Church. It is well known what Jerome states as the common opinion of earlier times.
And Ruffinus, speaking of the matter as not at all controverted, declares with Jerome that Ecclesiasticus, the Wisdom of Solomon, Tobit, Judith, and the history of the Maccabees, were called by the Fathers not canonical but ecclesiastical books, which might indeed be read to the people, but were not entitled to establish doctrine. I am not, however, unaware that the same view on which the Fathers of Trent now insist was held in the Council of Carthage. The same, too, was followed by Augustine in his Treatise on Christian Doctrine; but as he testifies that all of his age did not take the same view, let us assume that the point was then undecided. But if it were to be decided by arguments drawn from the case itself, many things beside the phraseology would show that those Books which the Fathers of Trent raise so high must sink to a lower place. Not to mention other things, whoever it was that wrote the history of the Maccabees expresses a wish, at the end, that he may have written well and congruously; but if not:, he asks pardon. How very alien this acknowledgment from the majesty of the Holy Spirit!

Based on this, it seems Calvin did approve of these books, but did not consider that doctrine could be derived from them.
PS: for a Catholic rebuttal to Calvin's criticism, see here.
PS2: I'm not a Calvinist.