That would be true if US passenger rail was actually capable of high speeds to start with.

As it stands right now US passenger rail is total joke compared to the rest of the developed world. Slower, less reliable, more expensive. Air resistance in the US is the same as everywhere else, so that isn’t the limit factor.

That is just one of the limits. At the speeds hyperloop was proposed to go, you need really wide curves to limit g-forces on the passengers. Not just when going left or right, but also up and down. It would be an absolute nightmare to get all the right-of-way done, and even then you would need immense amounts of earthworks to start curving uphill miles before the hill actually starts.

Let's walk before we run. Getting Shinkasen-style trains in the USA should be step 1.