You can easily sell a few Lyft shares. You cannot easily sell a billion dollars worth of Lyft shares because the market is not liquid enough for that. If you seriously think there's somebody out there with that kind of money just waiting to buy at current market price, please say who you think they are, because I believe they don't exist.
The major shareholders of Lyft are absolutely known, and that's because federal law requires disclosure from everybody over 5%. And I promise you that people who have or who are thinking about taking a serious position in Lyft pay attention to what they're doing.
An obvious example here is Musk's shenanigans with the Twitter acquisition. He was required to declare his stake publicly, which bumped the stock price. When things weren't going his way, he threatened to dump his whole stake at once, the implication being that the stock price would drop precipitously as he burned through the order book, causing much sadness among remaining investors.