I don't yet see if it is a correct good argument, though. I don't see a situation in which the user wants a fuzzy match for the merge. Every edge in the graph represents a diff that a specific user approved. This conversation is not about the fuzziness inherent in the production of those diffs, right? (That is a different but related issue so it can confuse discussion.)
So with this graph:
a / \ b1 c1 | b2
assuming that the user who generated the diff from a->c1 generated the diff they intended to, and the user who generated the diff from a->b1 did the same, and the user who generated the diff from b1->b2 did the same, then I don't think the user who asks for the merge of the two branches would ever want the fuzzy solution which ignores the a->b1 edge and the b1->b2 edge in favor of using just the a, c2, and b2 states.