Membership of the Administrators group for every server in the farm

If you use the Farm account (or known as system account), then it will have all the permissions it needs. Farm accounts normally will not have admin access to the SQL server, but db creator permissions within SQL. This is the necessary access an account will need as installation will create a sql database for Nintex.

Let us know if you have any additional questions

I remember Microsoft documentation say dbcreator+securityadmin roles on the SQL Server instance (not the server box) are necessary and sufficient. Has anything changed?

Nope! That's what you need and all that you need. You will not have to have admin rights to the server itself as I mentioned.