They have to say what code where is copied from the other project. It can't just be "parts", because that obscures the authorship.

You can open that PR, if you care to identify which parts were copied and label them all. Really, the people who copied the code in the first place should have done so, and really should have known better, given they work for a massive corporation that claims to love open source and has had a massive interest in copyright over the past three decades. It's not just a "mistake", it's unacceptable for a professional programmer for a corporation to take code from a FOSS project without crediting it. That's a level of incompetence bordering on malpractice for a profession that deals so heavily with copyright on a day to day basis.

edit: According to the MIT license, the notice itself just needs to accompany the code, so I was wrong about the specificity needed. Still, it does mean that any further forks would be unable to remove the license without personally identifying if all the original code was removed. It's always better to identify what code belongs to who.

The MIT license does not seem to dictate the exact location of inclusion. Logically, I would think you would want to associate it with the specific parts of code that you are copying. In the past, I've listed licenses together in the root license file for forks, and other times when the included code was a minor part of the overall project placed forked licenses within impacted files.