Well, it's based on that assumption if you're a leftist. Harris has spent the last two months tailoring her message to center-right Republicans, but if they vote for Trump or the Libertarian candidate, the centrist Dems aren't going to blame them. However, if the leftists that her campaign has been deliberately ignoring vote third-party, then they're entirely to blame.
Anyway, if you live in a swing state, you should vote for Harris, harm reduction, etc., etc. But if the Democrats are going to exclusively target centrists, it would be nice if they blamed the centrists for not voting for them.
Why not? The post is more likely to drive leftists away from the Democrats than towards them. Maybe these Russians have stopped posing leftists who are trying to convince other leftists not to vote Democrat and instead are posing as centrists in order to alienate leftists. There's just as much evidence for that theory as there is for any of the other baseless accusations of Russian trolling I've seen here.