The idea behind the protest is bringing him in so he can be tried. Either way, the LRA exists and he is head of it. Ergo, they are not merely just accusations, and the ICC must have substantial evidence already for him to be #1 on their list.
Actually, not entirely the case. The fact he is number 1 stems from the fact that the ICC Indicted him first as the first set of war crimes they dealt with were the ones in Uganda.
The numbering of criminals simply means that he is the longest outstanding indictment. Furthermore, as with many things the requirement for arrest are lower than the requirements for conviction. The arrest usually means there is a case with a chance of victory, whereas conviction means the case must be beyond reasonable doubt.
The LRA is a real organisation, but again the specific allegations have yet to be proven, no lead member of the LRA has been convicted yet to do with the alleged Uganda atrocities so there is yet to be a clear precedent making the case clearly guilty. No doubt with his arrest there will be evidence he can provide to the contrary.
Their arrest as I mention is welcome, I take issue with is saying that something has happened with certainty when a court has yet to find this the case.