Other than an interview from his father there's no evidence of it and Elon denys it. The mine was unofficial and there's no paper trail. It's a risky claim to make from a journalistic perspective.
Thankfully it's not journalism. It's comedy. I know it's hard to tell the difference, but here's a tip: comedians are far more likely to tell the truth these days.
Sometimes a mine just a hole in the ground with some valuable minerals. They paid locals to work it and sold the emeralds on the down low according to his dad, he had a partner. It wasn't a huge operation. But it paid their bills and then some.