He may be an investor of Cryptocurrency, you never know! Elon Musk is a smart man and a visionary leader. While the whole world is slowly acknowledging the power of Cryptocurrency, he may want to know about it with a future business model. Cryptocurrency is slowly getting into our daily lives and powerful leaders around the world are acknowledging it. It's hard to ignore!
I do believe he is an investor. But instead of telling "I invested in this", especially now, when big names call cryptocurrencies scam and bubble, he chose to make an advertising for his choice, but one that also tell people "read about it and you'll understand".