In my comparison of HSR vs. cars for <300 mile travel, the rail being 'drastically less flexible' means that it's subject to the same last-mile problem as planes are, whereas cars do not have this problem. Therefore the advantages of cars for trips like this are difficult for HSR to overcome.

As for buses, the article's author diminishes the significance of intercity buses in Europe by making it sound like private intra-national intercity bus service isn't competitive with HSR on travel time, as if HSR were widespread. HSR is only present along a dozen or so corridors in Europe, and while within France such premium bus services are a relatively new phenomenon, that isn't true elsewhere on the continent; so across the whole of Europe intercity buses are both much more common than he initially suggests, and much more competitive vs. rail than he suggests. After this, he does say buses thrive in the gaps between the train network, complement it, and have historically been important for international travel because of rail fare structures, and on those points I agree.

My wording of 'buses are flexible' does refer to the ease of introducing new routes (i.e. not having to build lots of rail), as a sibling comment correctly identified.