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.