Loose style is a very kind assessment of the way he plays. He struggles to stay in or find any kind of 'time'. It has to drive those guys nuts....or I bet they just have his mix cut completely out of their ears in order to just stay on their own time.
There is a quote out there from a fellow musician that was invited to watch a Metallica show from the soundboard. The sound guy offered him a listen to James' in-ear monitors. The only thing in James' own monitors was....himself. The musician wrote that James is literally the Metallica engine, keeping the songs on time and on track, which is amazing because it's usually the drummer doing that. But James is so precise and maybe hearing the other guys can throw him off.
So I have to believe that Lars just does his thing while James does the actual driving during the show. It works for them - Lars doesn't have to compete with any other drummers, he just has to be the best drummer for Metallica. I think his songwriting skills and business prowess have proved that over the years, even if he is just an average drummer.
I do love the videos of Metallica with Joey Jordison and Dave Lombardo from 2004 though, it's amazing to hear the band with so much power and precision behind the kit.
I don't have any problem believing that James has better rhythm than Lars.
With most bands, the other instruments keep up with the drums. In Metallica, the drums keep up with James.