It's not really possible to use no prefix in Flarum. Both the frontend (single page app) and backend routers are designed to work with prefixes to tell what kind of page is being requested.
There is this extension that allows changing the prefix to something different https://discuss.flarum.org/d/23872-custom-paths
Removing prefixes is not completely impossible but would likely require a lot of work, create incompatibilities with some extensions and decrease performance.
I'm not aware of any SEO benefit of shortening URLs like that. I would expect that to be a very minor criteria if at all.