isn't determinism rejected by Epicurus? How can Epicureanism and determinism co-exist in Onfray's mind?
You are correct that Epicurus rejected strict determinism. A primary point of contention Epicurus had with Democritean atomism was his determinism. If you have not come across it yet, I recommend reading Karl Marx's doctoral dissertation "The Difference Between the Democritean and Epicurean Philosophy of Nature" which addresses this point:
(Marx gets a few things wrong, but it is an interesting and somewhat surprising read)
Onfray positions himself in a long line of hedonists and materialists in general, but is not necessarily an Epicureanism in particular. However, based on your description, Onfray is not arguing for strict determinism, but rather, what we might call a compatibilist proposition, wherein determinism and indeterminism are not seen as being mutually exclusive.
While I try to avoid placing Epicurus within the buckets on modern philosophical categories, compatibilism might be the appropriate bucket in which to place him. Onfray seems to agree with Epicurus that "some [events] [...] happen by necessity and some by chance, and some are within our control" (Letter to Menoikeus).
Regarding "free will", we tend to look upon the concept with scorn, not because of the proposition of indeterminism, but because the concept of "free will" is a Christian form of (pseudo-)indeterminism that is contextualized within the domain of a Creator deity. The term "free will" is problematic for materialists, but not indeterminism, itself.