Coke and Nike don't directly deal with twitter tho. Agencies deal with all the delivery of adverts to the different advertising platforms. The agencies use what ever is available. They would sit and make dozens of different sized video files to fit different adverters specifications to upload into Facebook, Youtube, Twitter, etc.

That's only when the budgets are small that they pawn off the execution to some operations intern. For large budgets you have every chef in the kitchen on calls with the Twitter's Ad Sales manager who in turn will make sure all the assets work in the system.