I think it would have sold great either way, but the main reason it reached the level it did is because you've got a bunch of people stressed out over the virus and isolating and suddenly you have this super chill game taking place on a tropical island with a beautiful art style. Oh and you can play with a bunch of people online and chill together. My family literally had our sister-in law's birthday party on our island over the internet (we decorated and everything). Covid helped Animal Crossing and Animal Crossing helped people get through the shutdowns. How much it helped, who knows, but I'm inclined to think AC would have sold 5-10 million less (lifetime) had Covid not happened.
Covid did help, but Animal Crossing has an intrinsic appeal all its own and even besides covid there was more than enough stressful and depressing shit going down in the modern world by 2020 for it to be a hugely enticing dose of escapism. It still would've been a massive killer app had there not been a pandemic.
And I'm not arguing against that. I just said it would have sold 5-10 million less, which means it still would have sold 20-30 million copies (lifetime, my prediction) had Covid not happened, which still makes it a killer app. New Leaf sold like 20 million didn't it? That would be the basement for what New Horizons could do then in a normal market.