Is it possible WikiLeaks had many more e-mails that they didn't publish during the election to minimize the damage, and only release what was needed to shift public opinion?
If so perhaps somehow that one mail got included by accident, they removed it quickly, then disavowed it.
Now if JA is working with POTUS they're ready to do a proper full dump and this is a preview?
Who knows though one possible speculation, could be wrong.