Again, Harris was up in the polls when she was using successful tactics like targeting Republicans, calling them weird, and the slogan "We Will Not Go Back!" Those were popular, she had great momentum. And then she catered to the right, her polls plumeted, and she handily lost even the popular vote.
Harris and her campaign chose to run to the right of Biden in 2020. No promises of Student Loan Forgiveness, no Medicare For All, nothing, the only issue she took a hard stance on was abortion rights. She campaigned with Liz Cheney in the context of an ongoing genocide of Palestinians, this is a tremendous failure in understanding the optics of an election.
No, FRSO, PSL, Jill Stein, no Leftist or left-leaning boycott made a damn difference in the end. It all ended up being Harris failing to replicate the positive emotions surrounding the 2020 election with broad, popular policies. You have absolutely no evidence that fringe leftists somehow influenced people into staying home when Harris herself campaigned to the right of a successful Biden 2020 campaign. They tossed the winning formula into the dirt.