A lot of small things seem 'off' to me in this piece. After having gone through it, it should really say "in the US" because many parts of it around reasoning and attitudes come off as nonsense for someone living in the 'rest of the world'. It even feels like a "this is why things should stay as they are".
> airplane-competitive speeds, which for all but the shortest routes, require > 300 km/h or > 185 mph.
This number feels arbitrary, where did this come from, is it because very very few exist? Looking at the map, > 200 would do just as well and still lend itself to the author's point - some exist but not a huge amount. Even taking that point, high enough speed rail is good enough for inter-city travel, it isn't meant to be an aircraft replacement, it is an alternative form of travel. You wouldn't say that people shouldn't drive between cities, because airplanes are faster. It is an alternative form of travel which allows for mass transit, and in using it, you adjust expectations.
> Comfort and convenience are other important factors, but there aircraft are also quite competitive.
I doubt that, it's possible the author flies exclusively in business/first class, which is giving them a skewed perception. Aircraft are uncomfortable compared to trains.
> Contrast this with aircraft. There are 15,000 airports in the US. Any but the largest aircraft can fly to any of these airports. If I build another airport, I have added 15,000 potential connections to the network. If I build another rail terminal and branch line, at significantly greater cost than an airstrip, I have added only one additional connection to the network.
This is completely wrong. Building an airport doesn't automatically come with the ability to accept any aircraft. There are a lot of restrictions and regulations around what you can and cannot do with an airport and its associated routes. An "untowered paved strip" has a specific kind of usage which does not include commercial routes, which is what the premise of the post is, so this is a disingenuous, unfair, and misleading comparison.