Doesn’t that assume that Twitter actually knows that Elon is behind them? They could simply be other investors who have already decided to sell to Elon.
Sure, but that would be illegal and won’t happen. The SEC has filing requirements and the poison pill, if triggered, probably requires certain representations to be made (e.g. the buyer of the newly minted shares does not hold more than X% when aggregated with anyone they are acting in concert with).