Before we go too far down the rabbit hole of questioning the authorship of the letter to Pythocles, that translation of mine is a translation from the French of Les Epicuriens which "fills in" a lot of conjecture.
Here's the relevant section of PHerc 1005.
[ -ca.?- ἐρχόμενον ἀκριβεί-]
αι πρὸ̣[ς τὰ τῶν ἀνδρῶν],
[πε]ρὶ πολλῶν ἡγ̣[εῖσ]θαι [τἀ-]
κε̣[ί]νοις ἀρέ̣[σ]κοντ' , [ἐκ] τ̣ῆς̣ ἀ̣[ρ-]
5χῆς ὑποψί[α]ν τινὰ̣ [λ]α̣μβά-
ν[ει]ν ὡς περί τινων ἐπι-
στολῶ̣ν̣ καὶ τῆς [Πρὸς Πυ-]
θ̣οκλέα περὶ̣ μ̣[ε]τεώρων
ἐπιτομῆς καὶ τοῦ Περὶ ἀ-
10ρ̣[ετ]ῶ̣[ν], καὶ τῶν εἰς Μητρό-
δωρον ἀναφερομένων
Ὑποθηκῶν καὶ τῶν Μαρ-
τυριῶν καὶ μᾶλλον [δ]ὲ̣
τοῦ Πρὸς τὸν Πλάτωνο̣ς
15Γοργίαν δευτέρου, καὶ τῶν
εἰς Πολύαινον τοῦ Πρὸς
τοὺς ῥήτορας καὶ τοῦ̣ Περὶ
σελήνης καὶ τῶν εἰς Ἕρ̣-
μαρχον· ἐξέλεξεν δὲ καὶ
20[ ̣ ̣ ̣ ̣ ̣ ̣ ̣] γεγραμμένω̣[ν]
Sketched 1803-1806 by Giuseppe Casanova
Engraved 1844-1861 by Vincenzo Corazza
The idea that Zeno questioned the authorship of the letter to Pythocles is speculative at best, unnecessarily provocative to be provocative to be less charitable. I will say that, of I remember, On Piety (the famous Obbink translation one) is only ascribed to Philodemus on the basis of one initial Φ... in that papyrus and is conjectured to be written possibly by Phaedrus, the scholarch of the Garden.
Things get messy with old texts, but I see no good reason to question Epicurus as the author of the three letters in Diogenes Laertius at this time.