I'm not sure I got well your question, let me try. I understand you're a Core Circle Member in Circle A via a specific authorization process for being so. I also understand you do some audit/coaching activity on asynchronous proposals on circle A, as you said - which is an activity, right?
IMO one interesting question would be "Does someone consider such activity being inside the boundaries of the circle A"
My interpretation here is the following : If a Core Circle Member (either filling a role into such circle or not) considers that such activity is part of the Circle's work, then such Core Circle Member of Circle A may go to Governance to propose to add an accountability/role in order to reflect such activity.
So back to your question - "=> My understanding of "activity" is any action/project taken by any circlemember of the current Circle. Nothing else. Am I interpreting this right ?" - , I would say that an *activity* is work being done, and this work belongs to some circle and may be done either by a Circle Member of such circle or by another partner. Any Circle Member sensing a tension re such activity and considering such activity is part of the work to be done by such circle, may bring that to governance to more clearly reflect activity that is already happening.
-> See Section 3.2.1 "...Finally, a Proposal is always valid regardless of the preceding criteria if it is made solely to help evolve the Circle’s Governance to more clearly reflect activity that is already happening, or to trigger a new election for any Elected Role."