Yes, only core circle members can be elected as Secretary or Facilitator. My interpretation is that it does mean that a Partner may not fill only the Secretary or Facilitator role in a circle, unless they've been elected before when they were assigned to a role and were removed recently (but, in this case, are they core circle members? Good question!). The reasons I can see for that rule is that:
1- it takes time to attend meetings so more efficient to have one or more roles involved in the meeting
2- both roles need to be intimate with the governance records, knowing the different roles and authorities at play to interpret if needed (for the secretary) or to audit it (facilitator)
At the beginning, for new circles, we do recommend to have an outsider “neutral” though...