The article does not appear to deal with people at the nation-state level, but rather at the neighborhood level.
A large proportion of Singapore comprises of mixed ethnicities, even if they list themselves as Chinese.
That said, you won’t find a neighbourhood in Singapore dominated by a single community. Diversity is enforced there, because the government won’t allow a single ethnicity to dominate a region. In apartment complexes, you cannot typically have neighbours who share your ethnicity.