cosmo cramer,
Quoting from your source:
Maximum Endurance
(...)
Since the fuel flow is proportional to the power required, the fuel flow will be minimized at the point where the power required is a minimum.
For jet engines it is usually assumed that fuel flow is proportional to the thrust required.
Both are simplifying assumptions, as shown for the PW jet engine in the graphs I referred to.
You are right about two things though: the drag characteristics of a given airframe are not affected by the means of propulsion, and the fuel efficiency of a fan jet engine is somewhere between a straight jet and a turboprop.