Right now, the issue is that the officials of the commission that will be set up have not yet received the approval of the Burundian authorities to go there. If the authorities agree to receive the members of the commission of inquiry on Burundian territory, the members will clearly have access to the mass graves.
For the time being, there is no indication that Burundian authorities have allowed UN officials to access the mass graves on the ground. However, the existence of the graves has been proven. A number of reports by NGOs mention them and have identified their locations. So that's still a matter that deserves to be investigated.