This is addressed on a lot of grammar sites and i agree this particular usage is a murky area. I see your point and had considered that as well. But Grammarist covered it well with some incorrect examples and swayed me that it should be "defused." I've noticed that "diffused" seems to be a common usage in London. What else would you expect from those who confuse a "bonnet" for a hood, a "tyre" for a tire, and a "boot" for a trunk?
Grammarist: Confusion of defuse and diffuse is so common that we actually had some difficulty finding examples of diffuse used correctly. Here are just a few of the problematic examples we found: 1) It’s difficult to diffuse this much dynamite. [Times Leader]
2) Police negotiated with Finnegan in an attempt to diffuse the situation. [Cape Cod Online]