John McDonnell say himself that he is a Marxist. The current Labour manifest is hardcore socialist and many of their plans are exactly how things were in former socialist/communist countries.
It's clueless or disingenuous to claim otherwise.
You say their plans are "hardcore socialist", but most of them have comparable implementation in other western european countries. And socialist or not (socialism isn't a dirty word you know), they have generally been very successful for these countries.
To take one example, a large proportion of the British rail system that is supposedly "privately" run, is in fact run by the state rail companies of other countries (who bid on the contracts). It's kind of crazy that we allow them to profit off our country in this way.