It seems clear from this chapter that D'Holbach has deviated from Epicurus on the matter of "free will""
http://www.ftarchives.net/holbach/system/a11.htm
Last paragraph of that chapter:
QuoteThe false ideas he has formed to himself upon free-agency, are in general thus founded: there are certain events which he judges necessary; either because he sees they are effects that are constantly, are invariably linked to certain causes, which nothing seems to prevent; or because he believes he has discovered the chain of causes and effects that is put in play to produce those events: whilst he contemplates as contingent, other events, of whose causes he is ignorant; the concatenation of which he does not perceive; with whose mode of acting he is unacquainted: but in Nature, where every thing is connected by one common bond, there exists no effect without a cause. In the moral as well as in the physical world, every thing that happens is a necessary consequence of causes, either visible or concealed; which are, of necessity, obliged to act after their peculiar essences. In man, free-agency is nothing more than necessity contained within himself.
I don't have my sources on hand and I'm currently at work. But the conclusion I had with most of my readings of these middle Enlightenment era figures from France and Germany is that the despite the enduring popularity of Lucretius and the increasing anti-clerical sentiment, the rise of this "billiard board" model can be attributed to the profound influence that Newton had on physics and mathematics. We see a general rise in the belief of mechanistic determinism among these Enlightenment Epicureans, most prominently in La Mettrie.
On another note, the "riddle" showing up in Holbach is interesting. I don't know when Laertius was properly introduced, but one of La Mettrie's peers in the court of Frederick the Great owned and cited his copy of Lives and Opinions concerning Epicurus. Perhaps a foggy timeline of Lucretius became conflated with Epicurus' own time.