Are we talking about the same Mastery Learning as described in the article? Can you please explain why Mastery Learning requires teaching different grade level

Are we talking about the same Mastery Learning as described in the article? Can you please explain why Mastery Learning requires teaching different grade levels to the same class? My understanding is that all students study the same material, take a quiz, then the ones who aced it get to study whatever they want for the rest of the period and the ones who didn't ace it must listen to more lecture and discuss the content with the teacher and take another quiz. Where are the multiple grade levels of material? > it would require at minimum a complete reorganisation of how schools work. You would have to abandon age based classes altogether. Can you please explain this?