I feel like the explanation about the alpha parameter is a bit incomplete. The definition "The update speed parameter of the ISSM model for each item" is quite vague, when I think what needs to be understood is that alpha represents the correlation between one observation and the next one.
I would add that, the 0.3 value in the code example is way too high in most cases which can be misleading, a value of 0.05 would better fit usual cases to begin with.