Shane,
You are correct that an eligible clinician doesn't have to participate in an APM to be MIPS eligible. However, amsMipsEligibleClinician is a property provided by AMS (QPP's source for APM related information) and reflects a clinicians eligibility in relation to APM membership.
I believe the property you're looking for is mipsEligible, which is based upon a clinicians Claims data (and several other factors) and is not related to APM membership.
At this time, the Swagger API documentation is the best source of information for these types of questions. You're correct that it does not document certain scenarios (like the fact NPIs that are not part of an APM will not show the amsMipsEligibleClinician property). We are grateful for any input you may have to enhance documentation and training materials around the Eligibility API. Each recommendation gives us opportunity to improve down the road.
Please keep forwarding questions as needed
Craig
QPP Team