China [...] already has built one of the most advanced high-speed train infrastructure in the world, with more tracks than the rest of the world combined.
Well, to be frank, it was always more likely to do that than Liechtenstein, Monaco, Tuvalu or the Vatican City, for reasons nothing to do with relative technical, societal or economic advancements.
...ok, so that's a rather flippant comment. So now I'll chuck together some more expanded thoughts about why China isn't really a surprising holder of such a supremacy, even against the more practical competition.
That there's a basic elbow-room for such a record-breaking infrastructure, outwith the ability, need or will to create it, would more definitely the domain of countries such as Russia, Canada, China, USA, Brazil and Chile. Sorted by 'length', though whether Chile could usefully have significant spurs or loops of rail-links to outcompete its rivals. (By total length of all existing rail, those places are positioned in 3rd, 5th, 2nd, 1st, 8th, 30th place, respectively.)
By ability, arguably any country could scrape the budget to set up an HS link (assuming two or more somewheres to link between), given the right economic circumstances (either capitalistically or command-economy, and China's current hybrid system might indeed work in that favour), even if a bit of a White Elephant. Needwise, that'd be people-movements expected/required, above mere freight, and how people travel between various areas of cluster-density and others (US and its ilk skews away from that, Russia doesn't seem to be as philosophically desirous, Brazil seems to find alernate methods of hopping around more useful for either indiginously living peoples who already have their historic routes or the more 'cosmopolitan commuters').
So I'd say that China scrapes together the various pillars required to find itself the undoubted leader in its field. It can emulate/mirror other country's technological development with more space to expand into. Other countries have a travel-hungry population spread across large areas, but seem content to keep with what is basically the original infrastructure. Some countries are ambivalent about rail, and their high-speed projects were meager projects to begin with and get cut down further because of perceived boondoggling and/or political-mindchanging. (Japan, India, UK are 10th, 4th, 18th in existing rail-lengths, respectively, to punctuate my above example points.) China can carve out this space in the record books because it happens to have competitive factors across the board, but it could easily have lost out if any one of these circumstances were different (ok, so sheer geography is likely a fairly invariant factor, but a policy decision or two from the leadership could be the make-or-break).
That's not necessarily dependent of any achievements, or otherwise, in other exciting developmental metrics. China is definitely a current 'player', on the world stage, but I don't see high-speed passenger transport as a vital key to it. Helps some aspects. Arguably (e.g. with something like Covid, but I'm thinking of other aspects too) also has potent downsides to the internal situation that might make it a less useful investment. Time may tell if the barometer that has indicated past successes can be said to useful for future trajectories.
(China also has telecommunications as a target. Perhaps population helps it to be number 1 in telephones (land-line, mobile or both) and internet access (by numbers, can't establish comparative quality), and has clearly embraced this. Per-capita figures might show nuances, but with the drive to social-scoring everyone behind The Great Firewall I don't think there'd be any shying away from attaining competitive placing along the lines of Best Korea. Maybe this is what future assessments will find the most impressive key indicator, compared to any particular methods of travel (road, rail, air, suborbital!), or maybe even this will be considered subservient to some other metric.)