Pretty big assumption that "leftist" concessions would only appeal to self-identified leftists. Leftist policies happen to be massively popular. Ask any senior citizen if they want to give up their social security and medicare. Also, you'll notice that Harris had her biggest boost initially in polling when everyone was excited for the nominee to be NOT Biden. She has since continued to talk about how she's going to be Biden exactly, but less old, and her numbers have been steadily declining. It's clear that Biden's policies, (and not just his age) are important factors. People are eager for change - which almost always comes from challenging the status quo from the left.