I suspect that the reason Elon is investing in the Boring Co is to send a drill to Mars to create underground structures cheaply. This would create structures protected against radiation (and alot of space) relatively easily.
"I do think getting good at digging tunnels could be really helpful for Mars. For sure there's going to be a lot of ice mining on Mars, and mining in general to get raw materials. You can build a tremendous amount underground with the right boring technology on Mars. So I do think there is some overlap in that technology development arena."