Is this an error in the Italian Athenaze?
This sentence is in the margin on pg. 208:
Ὁρῶσι τὴν εἰκόνα τῆς Ἀθηνᾶς, ἐνοπλίου οὔσης καὶ Νίκην τῇ δεξιᾷ φερούσης.
And this fragment occurs in the main text on 209:
…καὶ τὴν τῆς Ἀθηνᾶς εἰκόνα, μεγίστην οὖσαν, ἐνοπλίον καὶ δόρυ δεξιᾷ φέρουσαν.
ἐνοπλίος means “wartune, march”. It seems like they meant to use ἐνόπλος instead, an adjective meaning “in arms, armed”.
ἐνοπλιος, ον, (ὅπλον) = ἔνοπλος, ἔρις Gorg.Fr.6; πρύλις Call.Dian.241; ἐπιστήμη D.H.20.2; πυρρίχη Anon.Vat.64: δρόμος Didyma201.14, neut. as Adv., ἐλέλιξεν ἐνόπλιον Call.Del.137.
II. ἐνόπλιος (with or without ρʼυθμός), ὁ, ‘martial’ rhythm, X.An.6.11, etc.; ρʼυθμὸς κατʼ ἐνόπλιον Ar.Nu.651; ἐ. σύνθετος Pl.R.400b; also νόμς Epich.75; ἀγωνία Phld.Hom.p.280.; ἐ. μέλη Ath.14.630f; Κουρήτων ἐ. παίγνια Pl.Lg.796b; θεῖν τὸν ἐ. Him.Or.2.20: hence ἐνόπλια παίζειν Pi.O.13.86.—On the ῥυθμὸς κατʼ ἐνόπλιον, v. Sch.Pi.P.2.127, Sch.Ar.Nu.651.
III. ἐνόπλιον, τό, contest in arms, of a race of war-chariots, SIG802A10 (i A.D.).
Liddell, H. G., Scott, R., Jones, H. S., & McKenzie, R. (1996). A Greek-English lexicon (p. 571). Oxford: Clarendon Press.
So you see ενόπλιος is really an adjective, just as ἔνοπλος is. So ενοπλιου is not actually wrong. (But I think you’re right that ενόπλου would have been better here, as being the more usual word for “armed.”)
Athena is conventionally portrayed as fully armed, since that’s how she was born from the head of Zeus! There are some nice vase paintings depicting the scene.