Rayner called for his resignation because he was being investigated not when he received the conviction as the article covers so your recollection was wrong.
The article isn't contemporaneous, but from last year and the editorial line is clearly trying to create an equivalence between Rayner and Boris' misdemeanours and to suggest Rayner to be a hypocrite.
But there really is no equivalence - on the one hand there was a criminal investigation into alleged criminal offences that occurred at no10, sanctioned by Boris in his capacity as PM, linked to legislation he, himself as PM, had brought in. And they were found to be true and Boris received a criminal conviction. In the other case the allegations had absolutely nothing to do with Rayner's public role as deputy leader of Labour ... and of course the investigation found there was no case to answer.
Oh, and Rayner said she would resign if she was found to have broken the law (she wasn't), Boris did break the law but refused to resign.
But hey, ho the PM gaining a criminal conviction for breaking the laws that he, himself brought in and an opposition MP who, following an investigation, was found to have no case to answer. Yup, absolutely the same - they are all the same, says NS so it must be true.