The way I've always thought of this question is this:
The author concludes that the titanium ink shows that Gutenburg wrote B-36, and shows that the Vinlandian Map may not be a fake as originally thought.
Taking each conclusion individually:
Evidence: We found Titanium in B-36 and in G's Bible
Conclusion: So, Gutenburg probably wrote B-36.
Assumption: Titanium ink was probably used only by Gutenburg.
Evidence: We found Titanium ink in B-36 and in G's Bible.
Conclusion: So the Vinland map may have been written in that period, after all.
Assumption: Titanium ink was probably used by other people as well as Gutenberg.
The argument assumes both that the ink was restricted to Gutenberg and that it was not restricted to Gutenberg.
This argument fits the old saying "you can't have your cake and eat it, too". If the map isn't a fraud, then other people were using titanium ink and thus someone else may have written B-36. If only Gutenberg had titanium ink, then the map is probably a fraud (Printers don't make maps; cartographers do).