Time delay calculation in System Identification Toolbox example

Hi everyone, in this MATLAB system identification example:

http://www.mathworks.com/help/ident/examples/estimating-simple-models-from-real-laboratory-process-data.html?refresh=true

Refer to Figure 3, I see that they determine the time constants (time delay) of system by counting number of output samples before the output goes beyond the confidence interval. I am concerned that what is the theory behind this method to determine time delay? Pls help, thanks!