I did not say that posted video was the exact solution, I posted that just as an idea of how multiple small attack vehicles can overwhelm any defensive systems.
What's the max ceiling for an S400? you can release the drones above that ceiling and let the gravity take them down to a certain height where they become active and attack from all directions, whether they are palm sized or an RC plane sized is not the issue, plus you don't have to take out all the missiles just focus on the tracking and search radars, take them out and missiles can be taken out by subsequent attacks by air assets or more missiles.
It's not easy, nothing ever is easy, but nothing is impossible either. Look at the evolution of all weapons systems beginning from simple planes with pilot dropping bombs with his hands to Ack Ack to early radars and missiles and counter measures. Its a constant never ending race and evolution. Every defensive /offensive weapon gives rise to an even advanced counter measure. The thing is you don't give up, you think outside the box, invest in R&D and come up with a solution.