What's unclear from this explanation is Step 5. "Evaluate one of the remaining columns (fifth column evaluation), and you're done." There are two remaining columns left, columns 31 and 32. So, how can we be done at that point?
For example, let's say your matrix has 32 columns and 8 rows.
You take the middle column, say column 16. You evaluate it and find that the column's global peak is superseded by an element to the right. You drop columns 1-16, and focus on columns 17-32 Find the middle column of the remaining matrix, which is column 24, and you evaluate for a global peak (this is your second column evaluation). You find you need to move to the right. Drop columns 17-24, focus on 25-32. Find the middle (column 28) - you evaluate (third column evaluation), and you find you need to move to the right. Drop columns 25 - 28, and focus on 29 - 32. Evaluate column 30 (fourth evaluation), find you need to move to the right, drop columns 29-30. Evaluate one of the remaining columns (fifth column evaluation), and you're done.