I regularly visit an area of Guangdong (a place called Baiwan, near Qingyuan, to the north of Guangzhou) where Black-throated Tit is one of the commonest and most easily seen birds in scrubby, lightly wooded habitats, and at no great altitude (about 200-300 m, and up).
Clearly it is trapped and caught up in the bird trade, but it is one of a number of species ('Grey-cheeked' Fulvetta being a prime example) which are so common, so wide in their habitat range and so plainly 'mobile', that it would seem to me incredible if it did not occur in HK from wild origins occasionally.
Not that that tells us any thing about individual birds, of course!
