That article makes it sound like the 'Date Closed' is just the date of the last payment, regardless of whether the invoice is fully paid or not. I don't think you'll ever get to a line level payment/non-payment status, since payment application happens on the record level and not the line (you can partially apply payments but just for an amount, not by a specific line). Looking at my data, I show all Vendor Bills as "Closed" even though the status for some of them is "Open", so I'm not sure the Closed value is even valid/useful, unless there's some weird interactions with our approval workflow happening. I would consider a bill closed when the status of the Bill is "Fully Paid". Alternatively you could look at whether the amount applied on all the applying payments = the amount of the bill.