@Jérémie Perez Thanks for reaching out. According to the documentation you provided, availability zones support is automatically enabled for the Event Grid Topic service. This means that you do not need to configure anything to activate availability zones for this service.
When you create an Event Grid Topic in Azure, it is automatically deployed across multiple availability zones to ensure high availability and disaster recovery. This is done by Azure behind the scenes, and you do not need to configure anything to enable this feature.
do let me know incase of further queries, I would be glad to assist you.