The Algolia extension does not rely on the anchor attribute. The products are associated to the categories with the attributes "categories" and "categories_without_path" in the product indices.
Algolia search results return products of a subcategory even if the anchor of the parent category is set to "No".
Anchoring categories in Magento allows products assigned to child categories to display within the parent category PLP and ensures that layered navigation elements display on that category's PLP..
Because layered navigation in Magento is essentially equivalent to the faceting features in Algolia, it's not necessary for the search experience provided by Algolia.
This category level setting in Magento is stored as is_anchor within the catalog_category_entity_int table in Magento and is explicitly excluded by our extension within the CategoryHelper object (\Algolia\AlgoliaSearch\Helper\Entity\CategoryHelper::getAllAttributes).
Because we exclude the attribute, the default behavior within the Algolia implementation is to show all products within a category regardless of where it appears in the descendant (child) category tree.