This makes no sense to me. Most talk about "recycling" space junk forgets that it's a lot like recycling bullets on a battlefield. The mass is there, but good luck catching it.
If all you need is mass, it might be more profitable to send a robot to a small asteroid and nudge it on its way towards us.
This only works for orbits of similar inclination, unfortunately. Shifting orbital planes is prohibitively expensive in terms of energy due to the conservation of angular momentum [1] and the rocket equation.
It might work for similar-enough orbits, though.
[1] https://orbital-mechanics.space/orbital-maneuvers/plane-chan...
Also, perhaps the most important thing to solve is a proof of concept - that we can remove any space junk. From there we can start to compare and refine ideas to figure out what the possible solutions are.
My biggest concern however is how they are going to keep the skyhook rotating. If it needs energy from earth the entire concept is kind of meaningless.