There are three types of magical items in this game. Focus items, enchanted items (including potions), and Items of Power.
Focus items help people cast spells, and have no effects beyond that. Obviously that's not appropriate for the Warden Sword.
Items of Power have abilities that can be represented as Powers that the wielder gains. Given an anti-magic Power like Might Over Magic (see the custom Power list), this could represent the Warden Sword quite well. But Items of Power cost Refresh, and I doubt Evil Hat wanted to charge Wardens Refresh on top of the standard Wizard Template cost.
Which leaves enchanted items. Enchanted items are items that can be used to create effects equivalent to spells. They're not a perfect fit for the Warden Sword, but they're good enough.
(Originally the Warden Sword was going to be an always-on item, I think, but those were removed from the game before release.)
As for the way that Warden Swords break the normal enchanted item rules, Luccio has special Crafting skills that normal Wizards don't have. And she made the Swords. It's not totally clear how item lending works for normal Wizards.