Lebron's argument is for longevity, he's been great for longer than anyone barring KAJ. I don't think anyone should dispute at the peak of their abilities, MJ was better. He was literally unstoppable, maxed out defender, maxed out offensively. His peak resume blows any player's peak out the water, it's almost like a fairy tail. And when you are considering "greatness", your resume and results matter. MJ is near perfection, while LBJ had a few bumps along the way.
Like I said, his case comes with longevity, if you favour the whole body of work, you can make a case for LBJ, personally I weigh heavier into peaks. If LBJ wins this year, his argument will actually be legit in my eyes, rn I think it's a bit of a reach to place him above MJ when he has so many losses on his record.