They don't test that directly. From what they report, it looks like the average is more accurate than the second guess, but not statistically significantly so. The average is 7.6 better than the first guess (with mean errors of 123.2 vs. 130.8, looking at all participants' first guesses, and the averages of only those in the dialectical bootstrapping condition). The second guess (of those in the dialectical bootstrapping condition) is only 4.5 better than their first guess, which is not reliably different from zero (95% CI = -1.0 to +10.4).

In the reliability condition, the first and second estimates for each question were nearly identically accurate, with a mean within-participants difference of 0.4 (SD = 6.7; Mdn = 0.0; confidence interval, or CI = 0.0–+1.4; d = 0.06). In the dialectical-bootstrapping condition, the second estimates were somewhat, but not reliably, more accurate than their respective first estimates (within-participants difference: M = 4.5, SD = 19.6; Mdn = 3.0; CI = -1.0–+10.4; d = 0.23).