The ambiguity in the statement primarily revolves around the relationship between Hester's scarlet letter and Dimmesdale's hidden guilt. The phrase "the same scorching stigma was on them both" suggests that both characters bear a mark of sin or shame, but it leaves open the interpretation of how that stigma is perceived and understood by others, especially the public.
Thus, the most ambiguous aspect is: It is unclear if readers are supposed to connect Hester's scarlet letter with Dimmesdale's covered heart. This ambiguity encourages readers to consider the nature of sin, judgment, and the hidden struggles of both Hester and Dimmesdale in their respective societal roles.