I guess the real question is "is Hong Kong trilingual" since most Cantonese speakers have a decent proficiency in either Mandarin or English - Better than we English expats can typically say.
As for why the level of English is 'bad', I do see a lot of snobbery and ignorance when expats discuss this. Why are we all so bad at learning Cantonese? Because it is HARD. why? Because the two languages couldn't be much more different. Grammar, vocab, writing system, vowel sounds, consonant sounds, tones, word roots, intonation, verb/ noun bias, child directed speech, Etc. Etc. Etc. Are COMPLETELY DIFFERENT in the two respective languages. It works both ways. Asking a Cantonese speaker without regular exposure to native English to pick up this completely alien language is not easy.
Completely different to asking a European to learn a European language, or even any non tonal language speaker to learn English.
Considering the difficulty and the time already consumed just to write their own ridiculously complex writing system, I think HK does a decent job, and we as expats who can't speak Chinese should shut up and admire them for the effort they've made.
I also think a lot of us, me included, cling to this concept of 'official language' to excuse our laziness or failure to learn Chinese. Plenty of countries have largely redundant official languages when compared to HK's grasp of English.
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