I don't. Unsure is lacking in support. They don't support him. They are included in the "less than half don't support" of the headline.
For another example of this language, atheist = lacking belief in a god. It does not mean you believe there isn't a god. Non-believers includes those who are anti-theist and atheist. They all lack belief, but one is against and one is neutral. Believers are the other group, who actively believe in one.