This is a stock response to mocking Musk's tunnel. And it's something "everyone" keeps asserting and it might be true (above my pay grade). But a lot of the "wow that was a cheap and fast tunnel" is due to the fact that the tunnel is really small, and some of it is due to the fact that when you're counting the costs of tunneling it depends on which costs you include. Cheaper is easy if you aren't too worried about the access points and minor things like ventilation shafts.
Infrastructure costs in the US are ridiculously high for various reasons but those reasons largely aren't technological and don't magically disappear just because Elon pretends they can.
But, yes, sure, if he's managed to build a better tunnel digger then good for him. I hope he finds a useful application for it.
