Singapore is de facto part of the Anglosphere, ergo its 'high level of English proficiency'. Singaporeans are effectively bilingual, and English is used at all levels of interaction, from the wet markets and supermarkets, to the public and private schools and universities, to the highest levels of government administration and communication. Can't get more 'Anglo' than that.
It's natural for the citizens of a country that uses English as though it were its native language to be proficient in it.
To add, I've read about Singapore's English proficiency before in their 2020 census report, which notes that 48% of households speak mainly English at home which is definitely very high and clearly shows that Singaporeans speak English.[0]
It's natural for the citizens of a country that uses English as though it were its native language to be proficient in it.