Hello Mark,
Yes, it is the same interface used to create the condition for both users and computers. Thank you for pointing this out, we will consider adding the information to the interface description.
As for the inactivity check itself, the Last-Logon-Timestamp / Last Logon and Password Last Set properties are used. For example, the If is inactive for more than 10 weeks condition will check whether the computer has its Last-Logon-Timestamp / Last Logon and Password Last Set older than 10 weeks.