That would help explain how the virus got from the mine to Wuhan; it does not help explain how the virus mutated from a bat virus unable to infect humans to a virus that was extremely good at infecting humans with, so far, no intermediate stages identified.
Much of the problem with the zoonotic explanation is that no evidence of the process has been found. Was a third virus involved? We haven't identified it. Was a third host species involved? We haven't identified one. Were there any half-way variants that were slightly capable of infecting humans, and then mutated further to reach their final form? We haven't identified any. There was some initial speculation about the wet market, but that hasn't panned out. Pangolins were briefly considered as a possible intermediate host, but I think that's also been ruled out.
So if the lab collected the virus, flew it to Wuhan, stored it, and then accidentally leaked it in its original form, we still have no idea what happened next. It's a bat virus, so it infected a local Wuhan bat, and then what? And whatever the theory is, why have we been unable to find any evidence of it so far?
Assuming RaTG13 was indeed the source of COVID-19, the question of "how it got to Wuhan" is, I think, much less interesting than "how did it mutate into COVID-19" (and "why can't we find evidence of that process?"). And I think when people talk about "lab leak" versus "zoonotic", they're generally focused on the latter questions.