I don't know if that's exactly the claim they are making, but I can certainly attest to their timeline. It was a day-night shift from "Bernie or Bust" to "Eww Bernie Bros are gross". Putting my own politics aside, I was more shocked by the sudden, very drastic shift in narrative. It's really hard to believe that it was organic. It wasn't even a "maybe we should support Clinton now that Bernie is out" narrative, which would have been more believable. It was a "Bernie supporters are sexist" narrative.
In the months leading up to the DNC primary, reddit was a battleground between Correct the Record[1] and /r/The_Donald. After the primary, it felt like every post on left-wing subs was inline with the narratives that CTR had been pushing for months. Before the primary, there was quite a bit of resistance to CTR, even from the left-wingers. That evaporated over a single night.
In the months leading up to the DNC primary, reddit was a battleground between Correct the Record[1] and /r/The_Donald. After the primary, it felt like every post on left-wing subs was inline with the narratives that CTR had been pushing for months. Before the primary, there was quite a bit of resistance to CTR, even from the left-wingers. That evaporated over a single night.
[1]: https://en.wikipedia.org/wiki/Correct_the_Record