How is it a myth? Whenever it's brought up, people flat out don't believe that he's deeply unpopular and blame the media for peoples' opinions of him.
Take his objectively terrible nonsense the other day about Remain vs. a fantasy Labour deal and how he wouldn't say how he'd vote in such a scenario - people still defended him.
The tribalism is insane when it comes to Corbyn - it's a cult of personality.
I don't think you've ever quite grasped what it is people like about Corbyn though. Nobody is/was drawn to him because of his huge personality because, as decent a person he seems to be, he's not exactly the most arresting public speaker is he?
The 'cult of personality' thing really bugs me, sorry. It's either ignorant or plainly disingenuous on your part to belittle someone's opinion that, hey, maybe it would be nice to have a genuinely socialist leader of the country's main socialist party? It's been quite a deviation since the last one, so they're quite entitled to defend it.
As for the 'objectively terrible nonsense' you mentioned – have you seen a more credible alternative yet?