Your understanding for `requires "fusion"` is correct as of now, but in future there are plans to start adopting semantic versioning for fusion and make it behave like any other 'normal' package. See <https://github.com/nim-lang/Nim/pull/16925>
I personally think that idea with automatically bundling fusion was not so bad, but since it is no longer how it works it would make sense to avoid any sort of special treatment for 'fusion-the-package', and make it work like any other.
