The Ecoboost is direct injected and direct injected gasoline engines sometimes have issues with coking. They build up a lot of carbon around the injectors and valves, which sometimes breaks loose and burns off.

Audi has had problems with this for years:

Audi FSI Engine Carbon Build-up Megathread

In some engines it can get bad enough to cause problems, but as long as the truck is running fine now I wouldn't really worry about it.