It isn't, though. Corbyn has been called a Marxist, and has said he admires Marx, but he has never come out and said "I am a Marxist", nor does a look at his politics over time suggest that he might be.
... because he's a politician who did what politicians do by dodging the label in an interview with Marr.
Boris Johnson doesn't have to say "I am a tit" to be a tit - he is one regardless. Same with Corbyn; he's blatantly a marxist; a follower of an ideology.