Yes, I guess you're right, but the maximum frequency is bounded by vehicle length and speed (you can't have a vehicle pass every thirty seconds if it takes more than thirty seconds for a single vehicle to pass a given point).
The longest TGV and Shinkansen consists are around 400 m, so they pass a given point in 30 seconds at about 50 km/h (30 mph). Of course there's spacing for safety and so on, but 50 km/h is so low that it's really a factor in practice unless you also have non-HSR traffic on the line.
A bigger factor in practical capacity is loading and offloading people and switching near termini.
