The kind of technical loophole that America's law system seems to be riddled with (and the "akshually" crowd kinda loves to play with)...
It's not technically wage theft, in spirit it's pretty similar to wage theft since the termination of employment contracts had severance payment as a clause.
> Severance was not included in Twitter's employee contract.
It likely was once promises were made to continue to pay severance for at least one year, and employees relied upon this promise (instead of immediately seeking to work elsewhere).