In the other articles you linked it's clear that IPCA could not identify the person who administered the kick. Presumably the independent witness on the scene could not.
In this instance it looks like the kind of case only video can resolve, for the purpose of identification and prosecution only of one of several individuals on the scene.
Other than that you just have to hope in the case the alleged offender is a complete psycho that colleagues will talk and eventually dob in if that continues.
I'm not sure what the alternatives were in this case based on the evidence available to the IPCA.
You're not getting it -- to the point where I can't decide whether you're just being obtuse or worse. The point isn't that the cops can't identify who did the kicking and can't hold the person accountable. Sometimes that happens. It's the fact that the police continues to either deny that the cops kicked him or are continue to offer some "alternative fact" scenario where the injuries were caused by the other lawful aspects of the arrest, after the independent watchdog (which on any rule of law and credible argument ought to be the body with the final say) ruled that there were unlawful and unjustified kicking to the arrestee's head.