Honestly, it doesn't matter if the tunnel ideas themselves are profitable. Phase 1 (make a better Bore) will make enough money on its own even if phase 2 never pans out, and Phase 1 is a very low cost risk by comparison. The key phrase in the interview is "Tunnel technology is older than rockets, and boring speeds are pretty much what they were 50 years ago."
Musk appears to be an efficiency hound above all else. Find something that sucks but has room for efficiency gains, make it better, and make money on it. He's done it with electric cars, batteries, and rockets so far. Making a better bore isn't that far-fetched. Making things more efficient in the right manner almost always leads to being cost competitive.