There's nothing conclusive so the DM decides
The DM decides either that a creature wearing bracers of defense is treated as wearing no armor for "to hit" adjustments for specific weapons or that a creature wearing bracers of defense is treated as wearing the armor and a shield appropriate for the bracers for "to hit" adjustments for specific weapons.
That's because there's this from the Dungeon Master's Guide (1977) on Weapon Types, "To Hit" Adjustment Note:
If you allow weapon type adjustments in your campaign please be certain
to remember that these adjustments are for weapons versus specific
types of armor, not necessarily against actual armor class. In most cases, monsters not wearing armor will not have any weapon type adjustment
allowed, as monster armor class in such cases pertains to the size, shape,
agility, speed, and/or magical nature of the creature. Not excluded from
this, for example, would be an iron golem. However, monsters with
horny or bony armor might be classed as plate mail if you so decide, but
do so on a case-by-case basis. Naturally, monsters wearing armor will be
subject to weapon type "to hit" adjustment. (28)
So in AD&D a monster not wearing armor just gets poked by a weapon much like a person not wearing armor. But the bracers of defense themselves actually say
These items appear to be some sort of wrist or arm guards. Their magic bestows an effective armor class equal to actually wearing armor and employing a shield. Of course, if armor is actually worn, the bracers will not be effective, but they do work in conjunction with other magical items of protection. (139)
Thus the DM may then rule that the "effective armor class" should be treated as the creature "actually wearing armor," making bracers of defense, AC 4 equivalent to chainmail and a shield. Of course, the DM could rule, alternatively, that effective and actually are meant to differentiate the bracers from actual armor, so that instead the wearer is treated as unarmored.
For what it's worth, a 2003 thread on the Dragonsfoot forums raised this exact issue. Many folks just didn't use the armor class versus specific weapons rules, but the majority that did use the rules used the AC 10 rules for bracers of defense.
Note that the AD&D 2E, the description of the bracers remains largely unchanged (DMG (1993) 162), but the rules for Weapon Types vs. Armor Modifiers (PH (1996) 120) become an optional rule that's even vaguer than in AD&D, AD&D, in this case, providing more guidance than its followup.