if DL is selling the I fare, this means that the flight is marketed as Business, not FC. You could say that I is like P in that business/BE fares are S I D C J, where S is special sale or deep discount available only on certain routes and I is normal discount or deep discount, depending on your perspective. Of course, FC fares classes are A P F.

If the 757 seat map shows 16FC, this would be true BE seating on an international configuration. More rows of FC and you have some form of a domestic 757 bird.