China's arming and training Rwanda, and the M23 uses advanced Chinese weapons, it's not that simple. This is more the US seizing an opportunity given to them by South Africa's failure, the US get to be a transparent arbiter of the conflict (US interests are transparent, the US wants natural resources and doesn't care about principles or morality, which in reality is a better offer than South Africa's incoherent morality based promises they have no ability to follow through on), freeze the frontlines (the official Rwandan army has to leave the eastern DRC, but M23 can stay, as they're not technically Rwanda and are bound by a seperate deal, so Rwanda still maintains unofficial control, but no further advance, the DRC must take action against anti Rwandan groups). And in the end, the US maintains control of rare earth minerals and cobalt in the DRC. China can't get them.
This is also one of the many ways BRICS is kind of a joke, Chinese made missiles aimed at South African helicopters and jets over mineral resources in central Africa. And then the US sweeps in and makes a deal.
It's stops further advance and the break out of a large scale war at least, without SADC or UN support, the DRC's odds are poor. The M23s seizure of Goma was swift and sends a clear message. All of North Kivu, and all of South Kivu bordering Rwanda, could fall under M23 control if the order is given by Rwanda. M23 is currently on the offensive in South Kivu.
Taking Goma was always going to be a big win for Rwanda. There's no undoing that. No South African or UN troops are going to die trying to re take it, it's gone.