People like myself say that to convey our intent to vote for Bernie in the primary election. It also conveys the sentiment that we dont really have any control over what happens if Hillarity manages to get her name on the ballot. We wont vote for her. Some say they will vote for any progressive alternative. Others say they will vote by write-in for Bernie. Many say they wont vote at all, that they havent voted in years and only came out to vote for Bernie anyway. Others even go so far as to say they will vote GQP. Not many are saying that though.
Regardless —— we feel if any other candidate gets in the White House it is a Bust. We dont see Hillarity as being any different than the failed Dimocrat party establishment of the past decades.
Oddly enough the Hillarity crowd started this nonsense long ago by insisting that if Hillarity wins the primary the rest of us HAVE to vote for Hillarity. At first I was among those who said there was no reason to look beyond the primary election to make decisions about the general election.
In typical conservative fashion the Hillarity supporters started demanding true Democrats pledge to vote for Hillarity. So most of us were forced to make it clear that we would Never vote for Hillarity.
When you push people to make a Pledge sometimes they surprise you. So the Hillarity crowd created the Bernie or Bust declaration.