I didn't really follow the Twitter takeover, so I don't know how big of a percentage the board that Musk negotiated with had. However, it's my understanding that different countries have different laws relating to this kind of thing. United is registered in the Cayman Islands, but of course the publicly traded stock is on the New York Stock Exchange, so I believe that the rules for taking a company private would be subject to Cayman laws, in addition to US laws. But exactly what those laws are, I don't know.
I do remember that when the Glazers did their hostile takeover of United, that once they owned a certain percentage, they could (or maybe even were required to) buy out the remaining shares, and there was nothing us PLC shareholders could do to stop it. I remember a big campaign by IMUSA/MUST to get fans to buy up shares to reach enough of a percentage that the Glazers couldn't force that purchase on us. But not enough people cared and that never came even close to happening. The US SEC laws probably have something similar, but I don't know what the percentage is.