and that you can create subsets and most importantly, union any two ethnicities together to get a subset.
ethnicity does not have that latter property though, so starting from the largest group and subdividing does not work.
"the americas is really big" is actually describing "there are many ethnicities in the americas that cannot be unioned together"
the union rule for ethnicities is that you can union two ethnicities and get an ethnicity only if there's significant overlap in genetics, language, culture, traditions, geography, etc.
there arbitrariness in setting that line, but its not approximately zero, as you've been suggesting, and we can tease that out with sniff tests. You maybe just dont know much about native americans, but you'd never put inuit and maya in the same ethnicity.
seriousness in debate and language would be to take curiosity in why the critiques think that size does matter, rather than asserting that it doesnt.
Ethnicity is arbitrary precisely because it is not clear what the boundaries of it are. I used Native Americans as an example, we all trace back to a few individuals in Africa. However Ethnicity is not just the genetics, it also includes culture, language, history, traditions and more. and it includes a component over time for example: "They have been living like this for x amount of years"(since after a war or similar displacement event) can be a divide in ethnicity.
You could subdivide ethnicity all the way until you reach the lineage and environment of a single individual. There are lots of ethnic differences between Native American tribes for example.