Homogenous is a pretty loaded term imo. You could have two genetically identical groups claiming they belong to different ethnicities or tribes and are thus, not homogenous. Or in the case of Japan you have two major ethnic groups, evenly disperses and so intertwined no one thinks about it, making them homogenous.
Homogenous thus in my opinion is more like a mix of homogeneity of appearance/culture. Indian people largely share the same appearance and culture, so in that regard, they are homogenous. Africans likewise. Whether some sub-groups have their own distinct appearance, or cultural practice, they are largely similar.
Unfortunately this (overfocus on genotype) is the mistake the west made when it came to determining homogeneity (and creating nationstates out of whole cloth) and it (plus other phenomena, not meaning to shift blame) has resulted in much of the ethnic cleansing of the 20th century.
As an example, the formation of African states, cutting across traditional tribal lines which resulted in the ethnic cleansing of the Igbo in former Yorubaland/Nigeria.
India has so many complex cases of this that covering it is an essay of its own.
Even Japan, with its claims of being an ethnostate, became one by force of ideology -- force so strong that it seems like common sense even though it was not true then or now.