They do define it clearly in the 45 page linked PDF. They look at 7 day standard deviation of bedtime, then separate it into 3 groups where the worst one is labeled irregular.

The standard deviation of the worst groups bedtime was 108 minutes.

“For bedtime regularity, the median variability was 33 min (IQR 13) in the regular group, 59 min (IQR 15) in the fairly regular group, and 108 min (IQR 55) in the irregular group. For wake-up time regularity, the corresponding values were 41 min (IQR 19), 73 min (IQR 15), and 114 min (IQR 41), respectively. For the sleep midpoint regularity, the median variability was 33 min (IQR 13) in the regular group, 55 min (IQR 11) in the fairly regular group, and 93 min (IQR 40) in the irregular group.”