Looks like it is Lucian I am remembering, so the issue would be the word in Lucian's Greek --
Lucian, section 47:
τὰς Ἐπικούρου κυρίας δόξας, τὸ κάλλιστον, ὡς οἶσθα, τῶν βιβλίων καὶ κεφαλαιώδη περιέχον τῆς τἀνδρὸς σοφίας τὰ δόγματα
So, you're correct. Lucian specifically uses κυρίας δόξας kurias doxas, simply the inflected form of Kuriai Doxai, and, of course, we have the title in the list in DL as Godfrey points out. Plus it works make sense for DL to include that book just like he included the letters.,