In a way, Nextcloud is a silo (even though it's open source and can be selfhosted). If you annotate documents there, you'd have to snapshot the page and move it

In a way, Nextcloud is a silo (even though it's open source and can be selfhosted). If you annotate documents there, you'd have to snapshot the page and move it into the app which kills the social function? In contrast, Hypothesis only keeps track of the metadata and uses fuzzy anchoring to make it resilient to the markup changes.