Maybe I’m missing some implication here. What do you mean by areas?
For example, suppose you work on the Search for Android team, under Search. You might work to train others in Search and others in Android as part of the work on Search for Android. Those don't count as citizenship, because they are directly related to your work.
You'd have to mentor people outside of that area, such as mentoring new hires in general, not as part of your job role.
That sounds like it would improve engineering, not water it down. Information spreading between people like a DAG, rather than a tree. Unless the "mentoring" is bulloney.