I'm not the one whom made that up, but I'll try to fill in Soresu.
Wes put up that a ship's hull armor composition, mass and density had something of a role in how well it performed when doing sublight maneuvers - it's listed as a speed increase, but it could include all sort of secondary factors such as handling, maneuverability, acceleration, etc...
We've four categories that portray a ship's resilience against harm. As you've observed, we've got three acknowledged armor types and one whom is not really considered so.
Steel and Titanium certainly do seem to fit the bill. Even though they can be considered fairly tough materials - present day helicopters, airplanes and boats can be made of steel and they aren't considered armored in that respect.
Another facet you might consider is a craft made of stealth material.
Finally, it's not obligatory to consider that the non-armored status is directly relevant to the material. Durandium being used does not make your craft 'lightly armored', if you have -enough- durandium layered protectively over your vehicle to represent armor, then you can pick up the light armor category. Especially thick durandium armor could just as well be considered medium armor too (with the extra mass and stuff slowing the ship down accordingly).
As a rule of thumb, I'd say dedicated stealth crafts and most non-military vehicles probably aren't armored. Usually when you armor something, it's because you expect it'll be shot at and hopefully survive the attack.