Fordhom
While a body cannot, I agree, generate a net force into "the flow" you have to be careful about "the flow".
The way the winglet can generate "thrust" (i.e. a force forwards along the direction of flight) is by taking advantage of the fact that the flow direction at the wingtip is NOT the flow direction of the gross air mass.
To take a very exaggerated example, if the wing tip vortex were so strong that it was essentially flow moving inboard and spanwise, then an airfoil pointed outboard could generate local airfoil "lift" which was normal to the flow locally, which would end up being forward in gross aircraft terms i.e thrust.
Now obviously the actual angles are not so large as stated there, but it is the case that I can use the angularity of the vortex to generate actual thrust.
If you wish, think in flow momentum terms. If the winglet acts to turn the vortical flow slightly more rearwards (which it does) then there must be an opposite reaction, which is a forward force on the winglet.
The diagrams in one of the links I posted show it better than words can explain.