Here you can discuss all things Ancient Greek. Use this board to ask questions about grammar, discuss learning strategies, get help with a difficult passage of Greek, and more.
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.