Boring a tunnel is rather different than excavating a plot around a house, don't you think? BTW, Elon has stated that pretty much everything with home construction is a field ripe for disruption, but that he already has too much on his plate.
At one point long ago - though I never got around to it - I had been considering building a geothermal heat store for my then greenhouse. Rather than excavating, my plan was to use an auger and drill 90° intersecting diagonal holes, to then slide pipes down into (with a 90° joint at the end of one of each pair of pipes, for the other to slot into). So you'd end up with a sort of "repeating V" pattern that goes underground, back up, underground, back up, etc. I figured that a bunch of auger holes would be one heck of a lot cheaper (and easier) than having to excavate a whole plot of ground to a significant depth. Never did get a chance to try that approach out, though.