I mean this actually makes a lot of sense to me in that in the US (my location) the majority of talking points, memos and even verbatim legislation is furnished by think tanks and 'public interest' groups with the money to fund and push their own agendas. Our politicians are already taught to repeat or support whatever copy they are given, without understanding the issue or implications. Critical thinking skills often seem at odds with the current political climate.
TL;DR: a campaign made its campaign documents (draft tweets)writeable to a large group of people, someone social engineered into the group and edited the documents