I remember that discussion well. It wasn't that Obama couldn't come up with it on his own, it's that he had shown no evidence of using it, either before or after. Ayers did use nautical references throughout his writings, and was a merchant seaman for awhile after college.
That's Occam's Razor territory.
That's what is so intriguing about this article. There are many things that were written about that this author implies needed special knowledge by the author (shibboleths), such as fluency in Hebrew. And then there are the names...
-PJ
Proving anything is going to be tough. If Obama and Ayers were that close, Ayers's phrases may have entered Obama's conversation and rough drafts without Ayers actually writing the book.