I'll leave this up I suppose, but I think this is very silly. It's essentially non-experts trying to do philosophical reasoning about other fields, badly.
Paul Cockshot is a (relatively good) materialist economist, but this is a pure mathematics topic of consideration. This isn't exactly the first time Cockshot has gotten away from his field of expertise and said some heinously ignorant shit. The paper he cites is this one https://www.scirp.org/pdf/OJPP_2016083016514850.pdf, written by a person credited as being part of Department of Psychiatry, NTT West Osaka Hospital, Osaka, Japan, which is awfully strange for a paper attempting to disprove a core argument in real analysis. Looking further and reading the paper, while the attempt to think in terms of the materialist interaction of the brain with the concept of infinite decimal expansions of numbers and limits is in some ways valiant, the mathematical conclusions that are drawn from it are nonsensical. The lack of citations is also telling; there's no engagement with any modern mathematics, or even a textbook on real analysis. Instead there are primarily references to Aristotle, Cantor's original papers, and a few other plucked articles that aren't particularly helpful. The posting from the journal at the end of the paper is also...concerning.
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This is not suggestive of academic due diligence.
There's nothing wrong or Idealist about the diagonal argument.