They say, outright, exactly what I've been saying: that the NUMBERS are right, but the numbers don't tell you what Bernie claims they tell you. They weren't able to dig up anybody to dispute the fact that Bernie's negatives would be guaranteed to rise in a general election. They weren't able to find anybody to dispute the fact that there's no statistical correlation in the past data on this. They weren't able to dig up anyone to dispute the fact that these polls don't even try to model general election turnout (which is historically dismal among the groups Sanders is winning).

Why they went with "mostly true", immediately after explaining why it's mostly false, is a mystery. But PolitiFact doesn't really have a very good track record with this sort of thing.