Most of these "overpopulated" stated have low carbon footprints

it's the globe itself that's overpopulated, not a specific area, demonstrated by the fact that the size of the population is having a negative impact on the habitat it's living in, this is what classifies a population as being overpopulated, there doesn't need to be a shrink in a specific population, the entirety of it should be decreased to a level that's manageable with our current living standards as you can't stop people from polluting apparently.

100 companies are responsible for 70% of climate change. How can you blame that on overpopulation?

it's not climate change that's causing the decrease in biodiversity, it's habitat loss, human activity and agriculture, these factors are also responsible for a large chunk of the co2 that's being produced and if they're not solved soon there's not going to be anything left to suffer from the effects of a 3C increase in temperature as the only thing that'll be left at this rate is lawns and crops.