I dont think a sell has anything to do with market cap because there is both a seller and a buyer. You are selling for 100,000 but someone else is buying for 100,000.
Try to read and understand before posting.
He clearly said he was selling at -5% the price!
Market cap is a really, really crappy metric but we seem to be stuck with it. It's purely the current price on an exchange multiplied by the number of all mined coins.
You could reduce Bitcoin's market cap by $50 billion by selling a few tens of millions of dollars worth of BTC in quick succession. The price is set by what's on the exchanges and there isn't a great deal on there to meet a giant sell.
Unfortunately, you can simply change the market cap by choosing different exchanges...
Just the difference from Coinbase and Bitfinex right now would make the market cap swing by 600 million.
Really useless metric, especially since it is obvious some coins will never be sols as they are lost forever.