Looking at https://en.wikipedia.org/wiki/List_of_countries_by_medal_count_at_International_Mathematical_Olympiad https://en.wikipedia.org/wiki/List_of_countries

Looking at https://en.wikipedia.org/wiki/List_of_countries_by_medal_count_at_International_Mathematical_Olympiad https://en.wikipedia.org/wiki/List_of_countries_by_medal_cou..., it seems that many neighbouring countries who had similar schooling systems perform similarly or better "per capita" (Hungary with <10M people, placed 5th; Romania 19M at 6th; Bulgaria with <7M at 8th; Serbia at 30th with <8M, Croatia at 42nd with <5M people — two of which should probably get some share of Yugoslavia's medals at 39th — there's also Serbia and Montenegro separately). So, Romania is at the sweet spot of high selectivity and population size to really pop out in these types of competitions, but it seem it's not really that unique as it's being suggested.