That one thing they denied being the right fo austrian investigator sto investigate in Serbia. So, in fact, they still tried to obscure the issue, they rejected the ultimatum in its most important point. AH was perfectly justified in its actions, while Russia and France supported a criminal, terrorism supporting rogue state.
Serbia did not, and it was proven the man belonged to the Blakc Hand terrorist group, and it was known the Blakc Hand was supported and funded by the Serbian government. Really, its exactly the same as with Taleban Afghanistan and El Kaida after 9/11!
The 10 points of the ultimatum, all of which were accepted except #6, were:
1. Suppress publications which incite hatred and contempt of the Austrian Monarchy;
2. Dissolve Narodna Odbrana (National Defence) and similar societies, confiscate their means of propaganda, and prevent the societies from reforming under new names;
3. Eliminate from public instruction in Serbia, both as regards the teaching body and the methods of instruction, all that serves or might serve to foment propaganda against Austria-Hungary;
4. Remove from the military service and the administration in general all officers guilty of propaganda against Austria-Hungary, names of which Austria-Hungary reserved the right to provide;
5. Accept the collaboration in Serbia of organs of the Austro-Hungarian government in the suppression of the subversive movement directed against the territorial integrity of the monarchy;
6. Begin a judicial inquiry against the accessories to the plot of June 28th who are on Serbian territory, with organs delegated by the Austro-Hungarian government participating in the investigation; (Serbia agreed to keep AH informed of the results of proceedings)
7. Immediately arrest Major Vojislav Tankosić and Milan Ciganović who were implicated by the preliminary investigation undertaken by Austria-Hungary;
8. Prevent by effective measures the cooperation of Serbian authorities in the illicit traffic in arms and explosives across the frontier and to dismiss and severely punish those in the Serbian Frontiers Service who assisted the authors of the Sarajevo outrage;
9. Furnish Austria-Hungary with explanations regarding statements from high Serbian officials both in Serbia and abroad, who have expressed hostility towards Austria-Hungary; and
10. Notify Austria-Hungary without delay of the execution of these measures.
Agreeing to those nine points was an enormous capitulation of one sovereign state to another. That the Hapsburg ambassador rejected the Serbian response and left the country
without consulting Vienna indicates that his instructions were to reject the response no matter what it was.
That is in no way comparable to Afghanistan and 9-11 where the Taliban wouldn't do anything at all. Read especially points 4 & 5 - these are very major concessions, and I'm not sure how #6 is "the main point" in comparison to these. The Serbians agreed to remove from their military and government all personnel that AH wanted!
The Serbian response to the ultimatum was more than enough basis for negotiation, and could have led to an international tribunal or some other solution to #6. Wilhelm himself was convinced that the Serbian capitulation had avoided war.
Even beyond that, even if you do reject the Serbian capitulation, it certainly doesn't justify invading Belgium, let alone France!