Over the last few election cycles we have learned pretty definitively that we know a lot less about electability than we think we do. Our last two presidents, Barack Obama and Donald Trump, were both viewed as axiomatically unelectable when they began their campaigns for the White House. Moreover, most candidates have both positive and negative and electoral traits so their overall electability becomes a post facto assessment rather than having any predictive value. For example, in 2016 Hillary Clinton was very electable until she wasn’t. Similarly, had John Kerry beat George W. Bush in 2004, the punditry would have explained that as a war hero with years of experience he was the perfect candidate, but he lost narrowly, so we have been told he was a northeaster liberal with limited charisma and therefore a week candidate. Given this, when somebody tells you they think a particular primary candidate is electable, what they usually mean is simply that they are supporting that candidate.
The presidential race has moved on and Sanders is no longer leading a political revolution, but is trying to remain relevant, and maintain some leverage, in a fast-moving election where fear of the Republican nominee has, as expected, helped solidify the Democratic base around Ms. Clinton and largely put to rest concerns about Sanders supporters refusing to support the former first lady.