On the above I see many opinions and hypotheses, but specifically no (even manual) tests or deeper analysis to support them, although they may be admissible, the backfly being not an orthodox way of flight. That said backfly is used in kite field.
Let’s take a look at it from the factual to the questionable. The backfly flight is fast as shown in the videos. It is a fact. Furthermore I could contend that the traction was almost zero. But I would say it is a feeling more than a fact. However, I am not aware that the backfly is used to generate traction. From there the question arises, how does this happen? What are the forces at play? Gravity? Maybe a negative lift? These are questions that only further tests and theoretical studies can answer.
A subsidiary question can be also: is the control secure enough? Suppose this is not the case. The other subsidiary question could be: what are the risks? Is a fall of the deflated wing on its trailing edge destructive? Would the possible loss of time during reboots destroy the power resulting from the overall density of the installation?
Then we would have to compare with what exists. To begin with, we must consider the inevitable time losses for the yoyo mode, as well as the losses due to traction, which are known and measured (see pdf above). Furthermore, what are the risks of crashing from VTOL mode to flight mode and vice versa? Are these risks increased due to the higher cut-in speed with a rigid wing? What are the consequences of a crash with a rigid wing?
Now the problem being the management of space and one of the possibilities remaining the vertical trajectory, the question can be: is there another possibility of making these back and forth always without turn (turns take space) but without backfly? The answer is perhaps, if the wing is reversible, able to fly from both sides, and moreover probably being limited as to its L / D ratio. Let’s take things further: can we manage space without vertical trajectories? Probably yes, with horizontal trajectories to start with, or perhaps much tighter figure-eights or Low radius loop.
There are also of course the rotary devices to be able to better manage the space, but they have another problem: they do not seem to be able to exceed a few meters of altitude, and this for years. I would add that if we used the kite lifter alone in crosswind yoyo mode, it would produce more energy than the rotary assembly.
Besides this, I don’t really believe in decisive ideas that would be hidden for years. When something works, we want to show it.