The rationale is probably that the people who care about his backstory will mostly find out about it anyway through word of mouth and social media. Going on about it in his leaflets would come across as sour grapes to people who aren't political anoraks, and probably lose him more votes than he'd gain.Driscoll will be getting my vote (he'd have got my partner's postal vote too, but we didn't twig in time that Driscoll was the guy Starmer had banned ). Interestingly, his leaflet doesn't mention any of that, just his record as Tyne mayor. That was probably a good idea, though not standing as Real Labour or similar does mean he'll lose the votes of some people who similarly missed his back story.
Also, being independent and letting his record and policies speak for him allows him to transcend left and right and pick up votes from people who are sick of all the parties. Being an independent voice for the North East and promising to improve public transport and deliver jobs can potentially take votes from Reform UK, advertising his Corbynism won't do that.