How is what he doing not conventional? That's a mail-order tunnel-boring machine he's got there.

And tunneling doesn't cost 1 billion dollar a mile. One Mumbai metro line is being built at 100 million dollar per km, the cost includes everything, stations, tracks, coaches, etc. And they're using 17 tbm for this purpose.

The cost is proportional to what kind of environment you're operating in. If it's ideal terrain it'll be cheap. If it's difficult, ever changing conditions with a high risk of subsidence damage to the structures above, get ready to pay way, way more.

See the Seattle "Big Dig" project for an example of how sideways things can go if you hit an unexpected snag.

From what I’ve read so far, they are making it electric, more powerful, faster, fully automated and continuous (no switching between digging and support-building).

A modern TBM is already exactly as you describe. They rotate pre-cast sections into place, then push off of them hydraulically to advance the machine. They don't stop digging.

Innovating on the TBMs is definitely part of it; I don't know why you are concluding from a picture that it isn't? Boring smaller tunnels is also part of it.

You can read more here: https://www.boringcompany.com/faq/

I don't have an easy way to check your claims against Elon's, for example whether existing "modern" TBMs are fully electric as you claim, or run on fuel as Elon claims, and whether continuous tunneling is the norm or not.