Because, as the article suggests, "There is nothing knowable that He does not know," the question comes "What is unknowable?" The answer seems to be, only those future things which are contingent on matters not within His control. From the standpoint of open theists, the only things not within His control are (i)the free will decisions of His creatures (to which God has, in a self-limited way, agreed to respect) and (ii) matters which are generated by other free agents, i.e. Satan.
It is not unusual that open theists (like Boyd) give a large swath to Satan. This accounts for the presence and persistence of evil (both 'personal' and 'natural') without in any way seeing God as the source of it. [In fact, Boyd refers to his theology as a "Trinitarian warfare theodicy."]
Thus, in sum, to the extent that these future decisions are truly independent of God, they are not knowable until they are made (and thus become 'facts' capable of being known) and therefore cannot be known -- even by God. It is this attribution of evil to the free agency of Satan and man that partially limits God's foreknowledge within the open theist view.
It is "partial" because God, of course, knows His own actions and, even as to those things which He does not 'know' (because they don't yet exist), He would probably have probability estimates based upon present knowledge of the decision-makers.
So, you can see how important this matter of (i) the sequentiality of God and (ii) the free agency of Satan and man can be to foreknowledge in the open theist view.