Does increased fineness ratio decrease skin friction drag?
Basically, your confusion comes from not knowing what needs to be held constant.  When you increase the fineness ratio without increasing wetted-area, i.e. you hold wetted-area constant, the overall drag goes down.  You can see this in racing shells (sculling).  The shells (boats) have an absurd fineness ratio, very narrow and very long.  This decreases the overall drag (somewhat, there are other forces at work there).

As you increase the finess ratio, at the expense of increasing the wetted area, you will need to do a trade-study to determine if you gain an advantage.  You probably will, up to a fineness ratio of 6:1.  Much beyond that the extra drag from the increase in wetted-area starts to dominate any savings in drag due to body fineness.

The inital statement is true, presuming you keep the wetted-area constant as you adjust the fineness ratio.

T