Yeah I forgot about the shield bonus with two-weapon defense. Now that I actually look at the item description I think the thread you pointed out is right as well as your interpretation.
Edit: Did you get the items recently Metavoid or were they on your characters when they had past approvals?
I'm not a reviewer for L4W, but a shield bonus of 0 doesn't exist. So, if a PC has a shield bonus, it will add to it. If not, it doesn't.
As rules support for this interpretation:
No shield, no shield bonus. No power, feat, or magic item, no shield bonus.
Note: one exception to this. If a PC had a shield bonus of +1 and a shield penalty of -1 to AC and/or Reflex, then yes, the item would work. Not sure if this is possible in the game system.
KarinsDad opinion...
A shield bonus is granted by your shield. Shield bonuses apply to AC and Reflex defense. Powers, feats, or magic items might provide a shield bonus; these typically help only characters who aren’t using shields.