What you quote above applies to RSUs, but that's not what Elon has. He has options.
When you exercise these options with a strike price of $X, and the current price of $Y, you have to pay the company $X, and you have made an instant income of ($Y-$X), and you owe tax on that amount immediately. As Elon pointed out, the number is so large that he's instantly in the top tax bracket which is >50% given the CA state tax. There are three ways to go:
1. Have enough cash/margin/borrowing available to cover both the company and the tax bill, and hold all the shares.
2. "sell" enough of the options to cover the costs associated with exercising all of them, and hold the shares that are left. This is called "sell to cover".
3. "sell" all of the options and keep the leftover cash.
I write "sell" in quotes because actually it's a three step process. First the broker lends you enough money to exercise the options and remit the tax, secondly the broker sells some or all of the resulting shares, and thirdly pays themselves back; it all happens in an eyeblink.
The equivalent to the 83(b) election is to exercise the options early, while their value is still relatively low, then hold the resulting stock. I have friends who were badly burned in the dot.bomb because the tax they incurred was actually more than the value of the stock after the bubble collapsed. Common wisdom is that if you believe in the company, delay exercise as long as is reasonable/allowable.
Aside: exercising the options is actually good for the company. It was carrying a liability on the books, and it gets to exchange the liability for $X in cash!