That is being reworked right now and Nextcloud also has file storage federation via the Open Cloud Mesh specification that allows sharing files and folders between various cloud file storage software. The latter is sadly often overlooked and more interesting than being able to post to Mastodon from the Nextcloud interface.
You are right! But there is a plugin for Nextcloud that allows to create and view posts like in Mastodon. (it is very underwhelming though)
https://github.com/nextcloud/social
That is being reworked right now and Nextcloud also has file storage federation via the Open Cloud Mesh specification that allows sharing files and folders between various cloud file storage software. The latter is sadly often overlooked and more interesting than being able to post to Mastodon from the Nextcloud interface.