Perth doesn't count as a major city? (% of that 30% live within its metro area. This isn't to say the 7-city grid is a bad idea, but your statement is inaccurate.
Perth (and Darwin) are essentally islands of their own.
Perth-Adelaide (nearest major metro) is 2,131km air mles, over the Great Austrailian Bight (ocean). Land route is 2,695 km (1,675 mi).
Darwin-Brisbane is even further: 2,848 km by air, 3,425 km by ground (2,128 mi).
By 300 kph HSR, those are 7+ hour trips, vs about 3 by air. And those are nearest-nighbour trips.
There is effectively no population between either location -- unless somehow induced (a possibility), a high-speed rail service would have tpo rely on endpoint-traffic only. Climate, economy, and ag productivity (low) make development unlikely.
By contrast, the southeastern zone of Adelaide-Melbourne-Canberra-Sydny-Brisbane, is compact with five major metros over 1,601 km (air), though the ground route is still 2,005 km (1,246 mi).
Giving mobility to those people would unlock new use cases of its own, but the GP post is actually accurate to commercial reality.