White-throated Laughingthrush is a montane west China species so highly improbable as a wild bird in HK. Even if it became established it would probably be treated as Category 2B (i.e. established exotic) like (e.g.) Blue-winged Minla.
Incidentally this species is on Cat 3 due to an old record of a flock on Tai Mo Shan.
Mike Leven