I don't know why he got fired from Amtrak.
But in fairness to him, he may have gotten disillusioned by the lack of progress in the US toward high-speed rail since when he wrote his book Supertrains, back in 1991.
In the meantime, nations on the east and west ends of Eurasia have made impressive expansions of their HSR networks, expansions that they continue to make.
Back in 1991, France had only two lines, Paris - Lyon and Paris - Le Mans / Tours, and was working on Lyon - Valence, Paris - Lille / Chunnel / Brussels, and a bypass around Paris. Now, France has lines to Marseille and Strasbourg with more on the way, the Brussels line has been extended to Amsterdam, and the Chunnel line to London.
Back in 1991, Spain was working on its first line, Madrid - Seville, and now, Spain has Madrid - Barcelona, Madrid - Valladolid, Cordoba - Malaga, and is working on several other lines.
Back in 1991, Italy had completed its first line, Florence - Rome, and now, Italy has a Turin - MIlan - Bologna - Florence - Rome - Naples line and the ends of a Milan - Naples line, with construction here and there.
Etc.
Some of the points he makes may be worthwhile, like spreading construction money too thin, but I think that that's a good political move. Spreading the money around helps produce a broad constituency. If all the Federal HSR money was spent on (say) the Northeast and California, then politicians in most of the rest of the US will be annoyed that they don't get anything. So spending on HSR efforts in the northern Midwest and the Pacific Northwest and North Carolina and Florida helps get more politicians to support HSR.
But pointing out low US population densities is an anti-HSR talking point that I don't recall him ever addressing in Supertrains. That is correct about overall population densities, but several US regions have much higher population densities than the national average, and most reputable US HSR proposals have been for such regions. I don't recall seeing any proposals for a NYC-LA TGV-quality HSR line.