I don't think amm14 was suggesting that anyone is ahead of the US. I think amm14 is saying that the second-ranking country is Switzerland, not Australia.
Not sure what source amm14 is using but the wiki page https://en.wikipedia.org/wiki/List_of_Grand_Slam_champions_by_country shows Australia with 41 (singles) as of today. The single titles by country are currently USA 138, Australia 41, (West) Germany 32, Spain 31, Switzerland 28, Sweden 25. If you include all the doubles too and count each player separately (as there are pairs from different countries) the order is USA 487, Australia 182, Spain 65, Switzerland 54, Sweden 50, France 47
I got a little excited when I saw Australia and cheered ever so slightly, before I realised this quiz highlights that we're second in these categories... Wah!
Suggest to change English (native) for English (first language). It could be argued that all countries with English as an official language have native English speakers. It's also the terminology used at the source (I guess) https://en.wikipedia.org/wiki/List_of_countries_by_English-speaking_population
https://en.wikipedia.org/wiki/List_of_countries_by_external_debt