As I understand it, they bought a very small TBM and dug some small tunnels cheaply, and then claimed that the cost is somehow comparable to the cost of big tunnels that have to be dug with big TBMs (which is obviously orders of magnitude more expensive).
The idea behind TBC is fine, which is to figure out how to tunnel as quickly as possible, with the assumption that the current methods are less than fully efficient.
The original TBC at the tunnel across the street from SpaceX was a proof of concept that used essentially off-the-shelf tunneling tech so that the team could learn how the current state of the practice was accomplished, and that they could improve on it.
I don't know if you're familiar with tunnel boring (and if you are, sorry I'm explaining something that you probably know better than I do), but essentially the phases of digging, mucking, and reinforcing are not done concurrently, and I think that was the first things they were trying to do, and then move on from there. But yes, smaller tunnels were part of it.
No, the idea behind TBC was to build a tunnel between Elon Musk's house (at the time) and the SpaceX factory for his private use, to shorten his commute from going around a mountain to going through it. That's why the "proof of concept" is where it is.
Also, they did not do any real innovation, as you seem to be claiming. Combining those steps is 50-year-old technology, at least, and has been done at scale (ie not on tiny tunnels) many times.
Is that TBM small enough to fit on a SpaceX Starship and send it to Mars?
I always thought Boring was a project intended for digging a base on Mars, fitting a whole Tesla in the tunnel would be even more than what's needed to connect some underground habitats.
