This negation is used sometimes for rhetorical effect. For example:
Doesn't the sunset look beautiful?
Does the sunset look beautiful?
Your example goes in the same category, where the negation adds a bit of rhetoric to the phrase.
Do you brush your teeth everyday?
You brush your teeth everyday, no?
So, to say, "I wonder if Diaz will be scared of the KO power," is a dry, clear, logical question with no bias. To say, "I wonder if Diaz won't be scared of the KO power" is like saying, "Diaz will probably be scared of the KO power", except the writer doesn't want to give such a strong assertion.