It sounded like a put-on. Musk is a serious person, but he can also be something of a loose cannon, making outlandish statements designed to troll the press or simply amuse himself. In a 2015 interview with Stephen Colbert he semi-seriously endorsed dropping nuclear weapons on Mars; last year he implied on Twitter that he's developing an Iron Man-style flying suit for the Pentagon. Most reporters assumed that the tunnel thing was another one of his jokes.
Musk wasn't joking.