I have heard that this story was supposed to be canon at some point but when did they decanonize it? Did it happen before the series was even finished? In the last issue of the miniseries, we see Bucky Barnes as an old man, that clearly makes this story automatically an AU and not 616.
I guess Marvel saw the negative reaction to the previous issues and decided to add Bucky to the last issue to imply that the series is non-canon.