I await a cordial, reasonable conversation from Atlas about this poll. 
Anyway, it does figure that Sanders would get a bump from his announcement, and it is one poll, but Harris should hope that these numbers are anomalies, especially the ones suggesting that a strong plurality of Biden's supporters would go to Sanders.
I was thinking the same especially since many of Biden's supporters are some of the same ones Harris should theoretically either do well with or target. Fortunately for her is that many Democratic voters are fluid and we have slightly less than a year until the primaries start.
Considering Biden could not be more different than Bernie, I'd find it hard to believe that a majority of his 2nd choicers would go to Bernie
I Think it's clear that those are just name rec voters. Note that this also goes the other way around. A quarter of Sanders' supporters say Biden is their second choice, those people can't know what they're doing either. I suspect a lot of Biden's support and probably a fair bit of Sanders' is pretty soft.
According to this, 51% said that Biden is among their top choices compared to just 38% for Sanders so you're right that support for Bernie is still soft right now.
