It's really not surprising. Bernie Sanders was in almost every respect a dark horse candidate: a socialist, an independent (albeit one that has caucused with the Democrats - he did not formally join the Democratic Party until November 2015), and one with no past reputation for seeking presidential office beyond a bit of talk here and there, in a city where talk is cheap. The most relevant part of that memo is not the end, but the start, to wit the date of May 26, 2015. The popular impetus behind Sanders' election bid was not apparent until June; it was not clear he was more than another Dean or Perot until even later. The reason this is relevant is because, at the time this was made, the DNC has precisely one candidate of merit as far as anyone knew: Hillary Clinton. The only person that might have challenged her broad political base of support was Biden, coming off his second term as Vice President, but at this point, it was not even clear if he would choose to run. At this point, the idea that Hillary would be the Democratic candidate was, though I'm loathe to use the term, "common knowledge:" one that generally lead to strong opinions on either side, perhaps, but the idea itself was not seriously challenged. As such, the DNC, with precisely one candidate in their party likely to win, decided to plan accordingly in order to put the Republican candidates on the defensive from the get-go. I hesitate to say it, but I'm not sure how shifty this is.