I notice that each post is set to be indexed by Google, this is the default flarum meta tags codes.
Indexing every single post permanent link in a thread is a very bad idea, especially if those posts are thin which are obtainable in forums.
The best approach by many forum software is indexing each page, a page may contain 20 posts or more. With this Google will not have to index thin contents. Indexing thin contents is a very bad seo idea.
Indexing thread pages instead of posts is likely not obtainable in Flarum since there is no pagination.