The world is not zero-sum but thats not really what he is arguing

Zero-sum socialism

The defining attribute of a zero-sum game is that it is redistributive: a ‘pie-slicing’ competition for a pie that will not grow. If we accept that, in the short to medium term, economic growth will likely be sluggish, then the task becomes to redefine the terms of the redistribution game – shifting it forcefully from identity to class.

What this should mean is, for starters, every MP, and every group or faction within the Labour family, sticking rigidly to universalist language and the rhetoric of nation, class, community and family

The British economy is not growing in per-capita terms, so in order to improve the lives of those at the bottom (as every social democrat and socialist should want) means redistributing from one group to another. If you do that along identitarian lines you are going to lose the white working class to reform, do it by class and you can benefit those same marginalised groups anyway, at least the majority of them who are working class.