There's nothing wrong with this argument. They're not literally saying the country is too big to fly a plane across, they're countering the argument made by bad faith actors that America is too big for trains, by pointing out that planes don't actually solve the problems people think they solve.
And they're absolutely right. I live five minutes away from a regional airport, and I would kill for high speed rail connecting me to some of the major cities in my neighbouring provinces. Hell, just a train from my town to the nearest city would be a godsend. People in North America will come up with any excuse to dismiss the potential rail has to really connect up a lot of disparate areas of these countries, because the reality is they just don't want to invest now in a solution that takes years to realise. If twenty years ago we'd all started building high speed rail the way China did, the difference today would be unimaginable.