Last bullet point on page 2 of the rules reference guide under the Abilities heading states "An ability that causes one or more figures to suffer damage(..) is not considered to be an attack...."
Okay. That's the rule I remembered reading. I just remembered it as the inverse. From this, I was definitely wrong. A stunned probe droid can self-destruct.
I'm actually a little disappointed that this resolved this way. I found it very thematic that the droid was stunned and unable to self-destruct. It added some strategy to the choice of attacks against it. Oh well. Now I know though. Thanks for everyone's input.