Earlier quoted context omitted.
Every jurisdiction is different in details, of course, but "at-will" just means you can fire someone, not that you get to decide what happens after unilaterally (e.g. you may owe severance)
Is that the case in CA or NY? I don't know where else Twitter has large offices, but presumably these would be two of the largest. I'm pretty sure in CA the employer doesn't automatically owe severance. It's usually offered in return for a promise not to sue (and sometimes also a promise not to speak ill of). There's the WARN Act, but I don't think that impacts what happens after a duly-warned layoff, unless I'm mist…
Technically the company can't get around the WARN act through severance but it is essentially pre-paying the damages so there's no reason to go to court.