At least he wouldn't be able to take McDonald's private as with Twitter.
Twitter's market cap (# shares X share price) hovers just under $40 billion.
Musk's net worth is said to be around $250 billion. Of course, that's not all liquid but one can see how he has the muscle to take on a relativity small company.
McDonald's current market cap (averaged over the last three quarters) is $188 billion. So, he would need to liquidate 75% of his net worth to gain complete control of McDonald's. That's why he put up that Tweet as a joke.
By the way, a few other market caps: Walmart $431 billion, Exxon $372 billion, Coke $286 billion, Microsoft $2 trillion.
Pretty soon you're talking about real money.