"The cost of a TBM doesn’t get much higher as you increase its diameter."

This is the key assertion, and it is unsupported.

- heat extraction will become a bigger problem

- debris extraction too

- cost of parts, cost and speed of logistics like transport and installation of the machine will be worse

- small number of exotic large TBMs will scale worse than large number of small TBMs in manufacturing cost and operating experience

- I've seen elsewhere (citation missing) that small borers move much faster than large so any costs that scale with time will be worse.

- risk-wise: a portfolio of small borers will have less risk than an all-eggs-in-one-basket gamble. a broken part delaying a big machine incurs more cost than ona smaller machine.

unless your project requires a single enormous bore I think you would prefer to use the smallest you can get away with.

The author does not seem to have any relevant experience, so I'm not going to take this on his authority; he's a programmer who has done scheduling software on metro projects and is working on an MBA. Interesting summary of types of machines though!