Nobody reasonable (I can find bad twitter takes too) thinks Starmer was trying to court the votes of that woman in LBC; indeed, he does say very clearly he thought the booing was wrong and racial justice is right. The issue is that by treating her politely he validates her.
I have some sympathy for the argument that if he engaged with her directly he'd have given her time to air out more of her views, which is clearly what she wants, but I don't think not addressing at all works either: she got what she wanted out of that interaction, legitimacy
The reasonable thing to do would be to immediately say "this is far right language" and hang up the call (and producers have a big responsibility here); the moment she gets to continue to be on air is a victory for her because she is engaged with as normal opponent in politics.
All in all it's a massive failure not only for Starmer in terms of a bloodlessness that he has demonstrated successively (failing trans party members) but also by everyone involved in the LBC phone in. She should never got in the air