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 wouldn’t seem to be in a particularly good ideological position to benefit from Trump’s infrastructure largesse. He’s a climate change hawk who was so closely identified with the Obama administration that Mitt Romney attacked Tesla during the 2012 debates. (Tesla had received a government-guaranteed loan in 2010.) In the next presidential election, Musk supported Hillary Clinton for president, describing Trump, in an interview with CNBC, as a man who “doesn’t seem to have the sort of character that reflects well on the United States.”