Nextcloud support has nothing to do with Logseq pushing their own sync solution. The issue is actually with Apple’s poor documentation around integrating with their files app. This issue has been known for ~3 years now but there hasn’t been a fix put out yet. You can find more information on the Nextcloud iOS app GitHub page:
TLDR: You need to add a specific line in your app’s code to properly integrate with the default files app. Without it, you aren’t able to open an entire folder from a different app, you can only open a specific file. This obviously causes issues since Logseq is looking to open an entire folder.
There are also existing requests for this feature. I would recommend upvoting and discussing there, as it will be easier for the developers to track community interest in a single thread rather than several disparate threads spread across the site
