I’m not 100% sure what you are trying to achieve. Do you want to add an extra filtering option on the activity list? This will involve some UI work as well as a call to bp_has_activities() template tag.
I think you might be getting confused between the bp_has_activities() template tag, and the bp_has_activities hook / filter.
It’s the template tag bp_has_activities() that lets you set ‘display_comments=stream’ as a parameter. I think you want to working with this template tag, and not the filter.
As a filter bp_has_activities accepts 2 parameters which you would then have to work with and modify (the list of activities, and a reference to the activities template).
