You're comparing a continent with a country. A person in Canada has very little in common with those in Mexico and/or Panama, etc. The same could be said about most continents.
GP is saying that if you're comparing to the US, it would make more sense to compare to e.g. Ethiopia (a single country with several ethnical groups and languages) than compare Malawi to Guinea (two different countries, thousands of miles away from each other)
Vermont and Idaho are also thousands of miles away from each other, a distance not achievable within Ethiopia's borders. I think people are missing the point, so to make it clear, in the US, you can move from Los Angeles to New York (not even extreme ends of the country) and experience the same culture, language, TV shows, cell phone, etc. at both ends. That would be the same as moving from Madrid to Moscow, or Lagos to Khartoum.
Sure but if you consider that you could say the same about e.g. Moscow/Vladivostok or Sao Paulo/Manaus or Sydney/Perth, it doesn't seem like homogeneity is that unique or impressive of a characteristic for an expansive country. IMHO it's more notable to point out that there's a language barrier between Toronto/Quebec despite them being only a few hours away from each other, a form of heteregenereousness that is also present in many countries in Africa, whereas in the US, there's a relatively high level of homogeneity even taking into account highly international places like Chinatowns because of the melting pot effect.