Hong Kong does in fact achieve the required density. THe stated cutoff is 25k/km2.
Kowloon's density is 43,000/km2. Hong Kong Island is "only" 15,000 but only a third of the island is built upon. HK Island is a bit smaller than the geographic size of "old" Toronto, over twice the population, and 2/3 the island is a nature reserve, or simply too steep to build upon.The urban areas exceed 40,000.
Even the New Territories, the traditional "Backyard" of HK, has a density similar to Toronto; clustered as a series of ~100-400,000 person "new towns" of very high density (Tung Chung on Lantau is planned for 27k/km2) with nature reserves and true rural areas between them. The East and West rail lines that serve them lie somewhere in that fuzzy boundary between metro and commuter rail; both lines run above ground except where tunnelled through mountains and where they terminate in Kowloon.
Even in HK there is no Sheppard-style suburban perimeter line. Everything's radial beyond the heaviest urban concentration.