It can pass through. There is even an official Authentik guide on the various methods specifically for Jellyfin: https://integrations.goauthentik.io/integrations/services/jellyfin/
Same with Authelia, though I don’t have a link for that on hand.
Unfortunately modrinth just doesn’t have the feature set and ease of use that curseforge does for creating, automating, and managing content. I don’t like it either, but until someone steps up and dethrones them as THE way to get content, they are the best platform, at least functionally.
The result of a mod creator uploading to modrinth and not curseforge is simply that their mod will not be included in all of the popular modpacks hosted on curseforge, which is a death sentence for download count and income.