Can Bell-lift distribution increase glider efficiency(L/D)?
Try to fly fast and you will see who hasn't understood the concept.
Just one datapoint: The lack of a rudder might be acceptable in a glider (but Heinz Scheidhauer, the pilot with most experience on Horten gliders, clearly said that at high speed the Horten gliders would be directionally indifferent) but not in powered flight. The fatal crash of the Ho-IX V2 can be attributed to loss of directional control after one engine failed.
On the SB-13 the vertical was replaced by two winglets which gave sufficient directional stability. However, when operating the plane, we often wished it had a conventional tail. Flight testing showed no clear advantage over current standard class gliders but operational restrictions were many.
All flying wings are susceptible to flutter because of their low pitch damping. In some Horten designs, flutter would limit the speed range at both ends.
Next: The low pitch stability forces you to use airfoils with low pitching moments. Trailing edge flaps can only be used in a very limited way, nowhere near what is usual on airliners. This limits the speed range almost as much as flutter does already.
And concerning the bell lift distribution: That lesson has been learned since maybe 70 years. Just look at the actual planforms: They are not elliptic and have little lift towards the tips.