I said I probably would, which is why I would've rather been laid off. I don't work at Twitter though, so I'm not going to resign my current job because of Twitter though.
You asked what was stopping someone (who wasn't fired but who wanted to leave) from just quitting.
Now, kenjackson's statement is kind of generalizing from self a bit. He believes he'd want to leave in that situation and believes that most people share his ideals in this case.
And while it's probably not the case, it's also not the case that the number is zero.
There are valid reasons to stay in an undesirable situation until you can line up something better.