How many Twitter engineers are on H-1B? 10%? 50%? 90%? Unless someone can find this number out for real, why is the default argument "they are all being held captive and cannot leave"? Would you apply the same logic to Tesla and SpaceX, who have the same working conditions? Here's a more reasonable take - engineers believe in the product they are building and are happy with the salary they get.
Facebook was reportedly ~15% H1B workers. It doesn't take a lot of skew in who was fired and who didn't leave for other employment for Twitter to be >50% H1B at the moment.