That’s sometimes the case, but also often it’s not the case. Mastodon allows multiple admin accounts, for example, and I know of many other applications where “admin” is a role that can be assigned to multiple users.
As far as I know, the “instance actor” in Mastodon is a distinct concept from admin accounts.
However, this potential ambiguity is why I think we may need an FEP for “instance actors” that clearly defines what they are and the requirements associated with them. The discovery technique could be part of that document or separate (especially if we envision alternative discovery techniques).