I know I don't say it was prime Big mom but at least she was probably back then high high tier/low top tier. We don't also know if Roger and Garp only fought as two against them. Probably they get helped by Rayleigh and co? We need more details about the battle.
Like
@Rmstorm said
BM having 500 millions when she is 12 doesnt mean she was high tier. At best Vergo level or Veteran since she is a natural freak.
Pre Ts luffy had 400 millions yet a a fodder to Pekoms or Pedro.
Xebec was the Emperor of his Era, this is why Roger couldn't do Jack without allying with Garp
38 years ago :
Xebec: Yonko of his Era. Basically the strongest Man at the time.
Garp : Admiral level at best (*around Fuji)
Roger : same as Garp
Rayleigh: Low top as Benn/Sabo
WB : Inbetweener as Kata or low top as Benn
BM : high high tier at best
Scopper : Veteran/low high tier
Kaido : Vergo/Base Sanji even tho he was a rookie