Question:

Why is a CRM user able to attend an Engage Meeting online without visibility to the Primary Account on the Engage Meeting?

Answer:

A CRM user invited to an Engage Meeting can attend an Engage Meeting online if they have visibility to the Call record for the Engage Meeting.

If a user does not have visibility to the Primary Account, they can join the Engage Meeting online if there are additional sharing rules or permissions providing them with visibility to the Call record.

Note: Offline, users require visibility to the Primary Account to attend the Engage Meeting. 

Related Documentation:

CRM Help Documentation: CRM Users Attending an Engage Meeting