I expect “them” to replace Harris before removing Biden.
How would they go about that? Only thing I can think of is somehow “persuading” her to resign. And if they had anything on her, we would have heard about it already, before the Biden documents broke.
If someone other than Harris were VP, I'd say the Democrats are moving against Biden to force him to step down. However, having Harris assume the presidency hurts the Dems more than it helps them going into 2024. I think the plan is to keep Biden in the White House, but damage him to the point where even he realizes that it's futile to run for reelection. They will then quietly back their chosen candidate in the primaries against Harris.