It's not about defending Starmer, it's about wanting the debate to be grounded in facts.
When the Mail accused him of being a multi-millionaire, they were trying to make him out to be a hypocrite. But the article was cobbled together in their usual fashion ( a source says...said to be ..estimated at...) and bore no relation to reality.
I've been linking the Evening Standard article as it is a relatively unbiased source, given that the ES has never given Labour a free pass. But as the multiple posters claiming he is a millionaire don't seem to be able to read it, here's a screenshot.