David
I think that even though each of these activities come under their respective Knowledge area, in this case, they are integrated with regard to the Project integration management.
An example would be 'Collect requirements' where all stakeholders are involved to provide their input but the process through which this is done is through 'Manage communications' (Execution) which is related or part of the Plan Communications management (Planning). The Plan Communications management is a subsidiary plan of Project Management plan (Planning), the latter of which comes under Project integration management.
It is the same with Manage stakeholder Engagement. The only way that is performed apart from Input, tools and techniques used is through the manage Communications process.
If I am wrong, perhaps, somebody may correct me.