I enjoyed the paper. The distinction between internal and external standards of ambiguity is fruitful.
Although the authors ask about "ordinary readers," it may help to figure out who the addressees of a statute are and then ask how these people would understand the text. This is the approach of using an interpretive community to identify meaning—the "community" part being important because linguistic conventions differ (think of the difference between how lawyers and physicians use language), while no one really knows who the "ordinary reader" may be. The more precise we are in identifying the interpretive community, the more likely we are to agree on how they understand a text, and thus what it means.
Unfortunately, a survey of first-year law students (or any other group) needs to hold down the length of questions and therefore does not include all of the information needed either to identify the right interpretive community or understand what tools that community would employ. For current purposes, let's suppose that the right community for criminal statutes is all federal judges serving at the time of enactment. Because the three statutes used as examples in the paper all are recent, we needn't worry about the difference between the interpretive community at the time of enactment and the interpretive community at the time of application.
Judges read statutes in context, not as isolated clauses. So for Chapman v. United States, 500 U.S. 453 (1991), the LSD case, the judges asked not whether the LSD's carrier medium (blotter paper) is part of the same "mixture or substance" as LSD in the abstract (or as a matter of chemistry), but whether LSD-in-blotter-paper is the same sort of mixture as cocaine-in-mannitol or other common dilutants. And the judges knew (as the students answering the survey did not) that the statute provided alternative punishment levels for pure PCP and diluted PCP but treated the weight of all other drugs as including the entire "mixture or substance". This implied that the weight of LSD's carrier counts.
Even so, every judge of the seventh circuit (which sat en banc, see 908 F.2d 1312 (1991)) and every Justice of the Supreme Court deemed the statute ambiguous. It's a surprise to me that so many of the first-year students found clarity where, despite the aid of context, the members of the interpretive community found ambiguity. What divided the judges was not disagreement about the existence of ambiguity but disagreement about how to respond to that ambiguity. Some thought it appropriate to use linguistic context to make a best estimate of meaning; others wanted to put the linguistic context aside and consider practical effects—which when coupled with the rule of lenity or a desire to avoid constitutional questions led to a vote for the defendant.
In a judge's daily work, there is substantially more disagreement about the consequences of ambiguity than about the fact of ambiguity. Textualists see ambiguity in statutes and regulations as often as pragmatists or intentionalists do (well, almost as often). The paper under discussion does not tell us much about how that ambiguity should be resolved. But it does caution against allowing one's own ideas of wise policy (the "internal judgment") to influence when a text will be declared "clear." For Ward Farnsworth reports (in his response to Einer Elhauge) that the internal judgment not only leads respondents to report less ambiguity but also leads them to treat texts as "clearly" meaning what the respondents think is the better policy. Judges can't be voted out of office, so they need an interpretive methodology that reduces the role of their own views of wise policy. No methodology can eliminate it, but the paper does tell us that using an external judgment will reduce the risk that judges will smuggle their own preferences into the law by a confident declaration of "plain meaning".