That is a whopping big misrepresentation of the presuppositionalist claim. The argument is not "we don't know, therefore God." Rather the argument is based on what we do know: laws of logic are universal invariant entities which govern all correct reasoning in this universe, and laws of nature describe and prescribe the relationships within this universe such that we can predict future states given sufficient knowledge of the present. What worldview can make sense of this knowledge? We find that only the Christian worldview can. THAT is the presuppositional claim. Any alternative leads to the absurd yet inescapable conclusion that we cannot know anything. Ironically, it is the atheist claim that is an appeal to ignorance: "There must be some naturalistic explanation for laws of logic / laws of nature, etc. After all, no one has proved the reverse."