Johnson is an interesting (interesting in the sense that it is probably the single worst one you could choose) example to pick as he has said many many dubious things over the years but never ever has he come under the scrutiny that Livingstone and Khan have in recent weeks.
Anyway, it really is the weakest argument. You can't find an actual example of hypocrisy so you invent one and say 'well you would say different if it was xxxxx'.