They are proposing putting the bubbles out at the Earth-Sun L1 point which is far outside where most of our current satellites are. Space debris is not really a concern there and I wouldn't put projectiles launched by a rogue state up there (there's plenty of closer things they would want to hit first).
There are plenty of inflatable space-based structures that have been proposed and some number of them tested (large inflatable antennas, inflatable elements to increase drag and bring space debris in, inflatable space habitats).
Granted, nothing is even within a few orders of magnitude of what they are proposing building so there are certainly still technical challenges.
Edit: Another technical challenge is the L1 lagrange point is not a stable point, so the bubbles will require some sort of control system (i.e. thrusters) to perform small orbit adjustments and remain in that spot.
I agree that space debris isn't too much of a concern if they have a very large grid of these bubbles. A percentage popping over a decade will be calculated in.
But L1 doesn't mean it won't get hit. James Webb has already been hit at its point in space.
There are plenty of inflatable space-based structures that have been proposed and some number of them tested (large inflatable antennas, inflatable elements to increase drag and bring space debris in, inflatable space habitats).
Granted, nothing is even within a few orders of magnitude of what they are proposing building so there are certainly still technical challenges.
Edit: Another technical challenge is the L1 lagrange point is not a stable point, so the bubbles will require some sort of control system (i.e. thrusters) to perform small orbit adjustments and remain in that spot.