Can we use Persons for approval flows without creating a user account

We have 20k employees who could all be responsible for approving invoices, this is because the manager of the Requestor approves an invoice.

There are actually only 100 users who will use the system for finance purposes.

Do we really need to create a user account for all 20k employees or can we just create a Person account for all but the 100 finance users and rely on the Person email to approve or reject invoice notifications via email?

Based on my testing where I created a person without a user account, I received an issue, below. When I then re-assigned that Requestors manager to a person with a user account, the approval flow worked. I assume it is not possible but looking to see if there is a solution?