I find it all pretty funny. Musk is clearly joking about, he's known for having a dark sense of humour. Twitter isn't a courtroom, it's okay to joke around. He's frequently said federal tax rates should be higher on more wealthy people, including himself, but the states keep refusing to increase taxes, so it's really not Musks fault he doesn't pay more, if Sanders wants the wealthy to pay more tax he needs to focus he efforts on his own backyard, the Senate.
On one side, yes it's stupid to suggest taxing people on "paper net worth" locked up in company shares and options, as this isn't cash, you can't pay taxes with shares, and to get the cash to pay the tax you have to sell shares which devalues the company, loses money for other share holders and reduces your ownership level of your own company. the other argument, which I do agree with, is the Senate need to do something to close the loophole that allows Musk to live a tax free personal life by borrowing money against his company shares, I believe that as it's seen as a loan, then it's not "income", but if you don't pay the loan back, or put it off indefinitely, then what's the difference?