I believe Button is just being realistic.
As I posted elsewhere, Alonso has been very consistent this year and it is quite reasonable to believe he can average 4th place each race for the rest of the season. If he does this he will be on 266 points.
To beat that, Hamilton would need three wins, two second places and a third place. Or alternatively, five wins and a retirement.
God forbid Alonso actually finishes on the podium in one or two of those races! In that case, even five wins wouldn’t be enough for Hamilton.