First lets get out in the open what madara admitts as his powers and what kabuto did and its effects separatly. Ill make this short and to the point.
1) Madara progression
Madara said he awoke the rinnegan shortly before his death.
But what was his situation during his final hours? Tobi(pretending to be madara) said he took hasihrama's dna during their fight. And Kabuto also said this, madara just starred at him silent, cop that madara u theif. Brushing off his powers as only his...lol
So did madara combine his ems with hasihrama's dna to prepare the awaking of the rinnegan later on as he knows of Nagato?
2) Kabuto and orochimaru reseach
Kabuto said he and orochimaru theorized that the sharingan can progress into the rinnegan.
So if madara already had the rinngean via unknown means what exactly did kabuto do recently to madara. The only real conclusion is that he made madara even stronger with more of hasihrama's dna from tobi to increase madara's chakra levels.
Some may say becuase of edo tensei that how muich chakra u have doesnt matter but its not. In terms of edo tensei's stamina yes it doesnt matter as you dont need to worry about running out of chakra. But this large metor technique has chakra requirements, Madara had the resrves to pull it off in one shot.
Regardless if u are edo tensei someone like lets say sasuke/itachi with their chakra capacity couldnt pull this off for one technique. That meteor jutsu would take alot chakra no matter who u are.