A mortage is a property loan from the French Death - Promise, however a property loan is not necessarily a mortgage, ie a loan to buy a property does not have to be a mortgage.
Yes, they are saying that any interest you pay on a loan realated to the property in question is offsetable against tax for any profit made on that investment. Essentially interest on a property you rent out is just another expense. Whether or not there is a mortgage dobument attached to the loan is irrelevant.