...If you, however, are financing a purchase (through a bank loan, let’s say), you will need to include the annual cost of financing your property as an operating expense...
The above is a quote from the blog. Annual cost of financing a property (i.e. interest expense) should not be included in NOI as it is not an "operating" expense. Interest expense is an expenditure related to the capital structure of the a rental business, not the operation of said business.
Now if you are in the business of lending and borrowing money, then that's a different story. In this case, interest expense would be an "operating" expense.