How do the HK controllers stop the distance from closing up?
They don't do it like that at all. Perhaps a naive attempt to 'prove' that Oz controllers are useless... Many HK procedures were developed by Ozzies who went there... As are many procedures in Dubai and many other busy international airports...
Time is a constant. 420Kts G/S (7 miles a minute) at 100NM and 5NM spacing is close to 45 seconds, between aircraft, if they fly the exact same profile they will land close to 45 seconds apart and at 150KTs G/S crossing the fence that puts them only 1.8NM apart; which even in HK is too tight! And certainly wouldn't want a 'chain' of aircraft doing that.