in no scenario would it be of any benefit for an upcoming developer to use Frostbite, which has no public documentation and no support from any source should you have issues. and no big company would ever use it because even the PR debacle if people found out they basically use stolen code would not be worth it.
so even if Frostbite was easy to use it would not be used by anyone outside of EA even if leaked.
even at EA internal studios there was a bunch of issues with the engine for some devs.
Let's say you have implemented a snowball effect. It works, but suboptimal. You are struggling. You get the leaked code, check how they do the snowball effect. Epiphany kicks in, modify your code, profit.