Bee-dubya-ell, that's certainly an important point in interpreting results. It's probably better to look at results comparatively, rather than taking any single number by itself.

On the topic, the report provides this note in the "FAQs About Measuring Religious Knowledge":

How did the survey account for people guessing at answers when questions were multiple-choice?

Guessing was possible, though interviewers subtly discouraged it, telling each respondent twice in the survey, "If you don't know the answer just tell me and we'll move to the next question." For all religious knowledge questions combined, a quarter of the time respondents volunteered that they did not know the answer, about the same rate as they gave wrong answers. The number of "don't knows" peaked at 71%, on a question about the Jewish philosopher Maimonides.

~ Becky