As far as I can tell, yes it's all based on RB's story to the police. That will change as they gather evidence, but police can't hold people for questioning for longer than something like 48 or 72 hours in most places, then they have to release the person without charge or go to a judge. The hearing before the judge, imo, was that hearing before the judge where they say there are accusations, and they feel they have reasonable cause to further question the people and either release them on bail to appear at a future hearing, or if there's too much risk that they will abscond, to hold them on a ridiculously high bail or without bail.
But I believe they are just preliminary charges/placeholder charges prior to further investigation, and that these charges come solely from RB's testimony, without any real bias from the prosecutor's office as to the actual details of which of them was actually responsible...that is something for the investigation.
They have a body, they have a confirmed DNA link between the body and that home. They know Rachelle's the mother, they have witnesses to say that MM was living with her, and he also admits this. They have an admission of some guilt relating to the death from Rachelle and an accusation from Rachelle that MM was the actual killer. That is all the prosecution stated in the hearing, and based on that information the judge decided on the bail amounts, which effectively meant both could be held in custody pending the next hearing on October 20th.